cmd/go: fix TestExecutableGOROOT when GOROOT_FINAL is set
If GOROOT_FINAL was set during the build, the default GOROOT
will not be testGOROOT. Determine the default GOROOT by reading
the right source file instead of guessing. (GOROOT_FINAL may no
longer be set when the test is actually run.)
Also refactor a bit.
Fixes #20284.
Change-Id: I2274595a235bee10c3f3a5ffecf4bb976f4d9982
Reviewed-on: https://go-review.googlesource.com/46428
Run-TryBot: Russ Cox <rsc@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>