dl/gotip: GOROOT_BOOTSTRAP always computed on windows

[maxatome]

Hi,

On windows gotip download always computes GOROOT_BOOTSTRAP, even if it already exists, using go env GOROOT:

https://cs.opensource.google/go/dl/+/5216546b:internal/version/gotip.go;l=152-158

In the case the go executable available in PATH variable is older than 1.17.13, the build fails with a message like:

Building Go cmd/dist using D:\some\path\go. (go1.15.15 windows/amd64)
found packages main (build.go) and building_Go_requires_Go_1_17_13_or_later (notgo117.go) in C:\Users\runneradmin\sdk\gotip\src\cmd\dist

Of course PATH variable can be modified just before launching gotip download to prepend the path of a more recent go, but shouldn't it be cleaner to use GOROOT_BOOTSTRAP to solve this? and so to compute GOROOT_BOOTSTRAP only if it is not already defined, as in:

https://cs.opensource.google/go/dl/+/5216546b:internal/version/gotip.go;l=152

-	if runtime.GOOS == "windows" {
+	if runtime.GOOS == "windows" && os.Getenv("GOROOT_BOOTSTRAP") == "" {

This way we can define GOROOT_BOOTSTRAP, before launching gotip download, on windows as we do on other platforms, without requiring PATH manipulation specifically on windows.

[bcmills]

@maxatome, it sounds like you have a fix in mind. Want to mail it?
(https://go.dev/doc/contribute)

[maxatome]

Sure @bcmills, I just wanted to be sure it was the right way to go :)
https://go-review.googlesource.com/c/dl/+/497335

[gopherbot]

Change https://go.dev/cl/497335 mentions this issue: dl: don't overwrite GOROOT_BOOTSTRAP if already set on windows