Fix compile --build-path not working with relative paths

[silvanocerza]

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• The PR has no duplicates (please search among the Pull Requests
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• The PR follows
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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

Fix a bug in a command and relative gRPC interface.

• What is the current behavior?

Calling arduino-cli compile --build-path ../build-path -b arduino:avr:uno ~/Arduino/Blink fails with an unclear error cause of the relative build path:

Couldn't deeply cache core build: Rel: can't make ../build-path relative to /tmp/arduino-core-cache                                                
Running normal build of the core...                                                                                                                
                                                                                                                                                   
Error during build: Rel: can't make /home/alien/workspace/build-path/core/core.a relative to ../build-path

The related gRPC function has the same behaviour.

• What is the new behavior?

Using a relative build path doesn't cause issues anymore and it's handled without problems:

~/workspace/arduino-cli
$ ./arduino-cli compile --build-path ../build-path -b arduino:avr:uno ~/Arduino/Blink
Sketch uses 924 bytes (2%) of program storage space. Maximum is 32256 bytes.
Global variables use 9 bytes (0%) of dynamic memory, leaving 2039 bytes for local variables. Maximum is 2048 bytes.

~/workspace/arduino-cli
$ ls ../build-path 
Blink.ino.eep  Blink.ino.hex                  Blink.ino.with_bootloader.hex  compile_commands.json  includes.cache  preproc
Blink.ino.elf  Blink.ino.with_bootloader.bin  build.options.json             core                   libraries       sketch

• Does this PR introduce a breaking change, and is
  titled accordingly?

Nope.

• Other information:

Fixes #630.

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