cmd/go: use os.Executable and drop GOROOT instructions

[crawshaw]

A common refrain on the Go mailing list is "don't use GOROOT". It is good advice, GOROOT leads to confusion.

However, right on the Go download page, https://golang.org/doc/install, users are instructed to set a GOROOT: "In this case you must set the GOROOT environment variable to point to the directory in which it was installed."

I sometimes give users alternate instructions about how to use a copy of the binary toolchain they just downloaded as GOROOT_BOOTSTRAP to rebuild the toolchain in its new location, so that setting GOROOT is not necessary. But this is several extra steps of work that are mysterious to a new Go user.

As an alternative, Go 1.8 introduced os.Executable. The cmd/go binary could use this to find its own GOROOT, and do the right thing by default if the distribution is installed to an unusual location.

WDYT? @rsc @ianlancetaylor

[crawshaw]

Ah, my first github issue search was bad. This is a duplicate of #18678.