build: default GOROOT_BOOTSTRAP to go env GOROOT

[vcabbage]

Currently, developers wanting to build Go from source need to either have a Go install at $HOME/go1.4 or explicitly set GOROOT_BOOTSTRAP. I frequently recommend prefixing the build scripts with GOROOT_BOOTSTRAP=$(go env GOROOT), this has worked well in the cases I've helped with. I believe baking this into the script would simplify the process for many developers.

To retain backward compatibility, I suggest the following precedence:

1. Explicitly set GOROOT_BOOTSTRAP
2. $HOME/go1.4
3. go env GOROOT

[ianlancetaylor]

Good idea. I'll note that if go env GOROOT == GOROOT (that is, the GOROOT we are about to use for the build), then we shouldn't use it.

[crvv]

This is a duplicate of #14339

[gopherbot]

Change https://golang.org/cl/57753 mentions this issue: make.bash: add go env GOROOT as default GOROOT_BOOTSTRAP value

[mvdan]

@crvv if this is indeed a duplicate, this issue should be closed and the CL should only mention the original issue.

@vcabbage could you confirm this is a dup?

[vcabbage]

It does appear to be a dup. Though #14339 specifically refers to make.bash, I would hope that the other make scripts would be updated to follow the same logic. I'll go ahead an close this issue.