[skip changelog] Improve install script's check for conflicting installation in $PATH

[per1234]

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• The PR has no duplicates (please search among the Pull Requests before creating one)
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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

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• What kind of change does this PR introduce?

Bug fix

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• What is the current behavior?

The installation script's check for a conflicting prior installation in $PATH was prone to false positives. For example, any of these examples result in a spurious "An existing arduino-cli was found at..." error:

• Paths differ by a redundant slash in $PATH:

  $ export PATH="$PATH:$PWD/bin/"
  $ curl -fsSL https://github.com/raw/arduino/arduino-cli/master/install.sh | sh
  ..
  An existing arduino-cli was found at /home/foo/bin//arduino-cli. Please prepend /home/foo/bin to your $PATH or remove the existing one.
  Failed to install arduino-cli

• Paths differ by a redundant slash on custom installation path:

  $ export PATH="$PATH:$PWD/custom-path"
  $ curl -fsSL https://github.com/raw/arduino/arduino-cli/master/install.sh | BINDIR="$PWD/custom-path/" sh
  ...
  An existing arduino-cli was found at /home/foo/custom-path/arduino-cli. Please prepend /home/foo/custom-path/ to your $PATH or remove the existing one.
  Failed to install arduino-cli

• Use of relative paths:

  $ export PATH="$PATH:$PWD/relative-path"
  $ curl -fsSL https://github.com/raw/arduino/arduino-cli/master/install.sh | BINDIR="./relative-path" sh
  ...
  An existing arduino-cli was found at /home/foo/relative-path/arduino-cli. Please prepend ./relative-path to your $PATH or remove the existing one.
  Failed to install arduino-cli

• Symlink:

  $ export PATH="$PATH:/home/foo/symlink-path"
  $ ln -s "$PWD/bin" "/home/foo/symlink-path"
  $ curl -fsSL https://github.com/raw/arduino/arduino-cli/master/install.sh | sh
  ...
  An existing arduino-cli was found at /home/foo/symlink-path/arduino-cli. Please prepend /home/foo/bin to your $PATH or remove the existing one.
  Failed to install arduino-cli

When the check for conflicting installation in $PATH has a positive, a "Failed to install arduino-cli" error message is displayed, even though the installation was successful.

When the check for conflicting installation in $PATH has a positive, the script has exit status 1, even though this positive does not really represent an error.

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• What is the new behavior?

The check for conflicting installation in $PATH is made more robust by comparing the resolved, absolute paths.

When the check for conflicting installation in $PATH has a positive, no confusing "Failed to install arduino-cli" error message is displayed.

When the check for conflicting installation in $PATH has a positive, it prints a helpful warning, but the script's exit status is 0.

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• Does this PR introduce a breaking change?

The script no longer returns exit status 1 when a conflicting installation in $PATH is detected. This is a breaking change for anyone relying on the previous behavior. However, I think this is unlikely.

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• Other information:

If you want to test the modified script using the recommended installation method, the command is:

curl -fsSL https://github.com/raw/per1234/arduino-cli/fix-install-script/install.sh | sh

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My initial instinct for improving the comparison was to use the -ef file test operator, but that is bash-specific while the install script specifies sh.

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It didn't end up being relevant to this work, but I'll add the information here for future reference since I had trouble finding it: The original base for the install script is here: https://github.com/Masterminds/glide.sh/blob/master/get, not in the repository mentioned by the comment in the script.