cmd/go: GOMIPS environment variable does not affect caching of mipsle code [1.10 backport]

[gopherbot]

@FiloSottile requested issue #27260 to be considered for backport to the next 1.10 minor release.

@gopherbot please file this issue for backport to 1.11 and 1.10 (GOMIPS and the cache were introduced in 1.10, so it might affect both, if not please close the 1.10 issue).

@ianlancetaylor had milestoned this for 1.11.1 and I don't see a cherry-pick, but it was closed by the master fix.

[gopherbot]

Change https://golang.org/cl/146560 mentions this issue: [release-branch.go1.10] cmd/go: add GOMIPS value to build id for mipsle

[gopherbot]

Closed by merging 15c6cc7 to release-branch.go1.10.