cmd/go: cache coverage profile with tests

[awartoft]

This CL stores coverage profile data in the GOCACHE under the
'coverprofile' subkey alongside tests. This makes tests which use
coverage profiles cacheable. The values of the -coverprofile and
-outputdir flags are not included in the cache key to allow cached
profile data to be written to any output file.

Note: This is a rebase and squash from the original PR #50483 that was
created/closed by @jproberts that I plan to maintain.

I don't know if anyone has considered the environmental impact
(Yes, of course, dev experience too), but on a team with 3 backend
developers, when I replaced our CI Golang version with this build,
it reduced the build time by 50%, which would have
equated to about 5000 hours of CI reduced in the past year.

Fixes #23565

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Patch Set 1:

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This PR (HEAD: dd0c9d9) has been imported to Gerrit for code review.

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Message from Antoine Hedgecock:

Patch Set 3:

(1 comment)

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Message from Bryan Mills:

Patch Set 3:

(7 comments)

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