cmd/go: test cache uses stale coverage data with -coverpkg [1.26 backport]

[gopherbot]

@matloob requested issue #74873 to be considered for backport to the next 1.26 minor release.

@gopherbot please backport

[gopherbot]

Change https://go.dev/cl/764381 mentions this issue: [release-branch.go1.26] cmd/go: invalidate test cache when -coverpkg dependencies change

[gopherbot]

Change https://go.dev/cl/764542 mentions this issue: [release-branch.go1.26] cmd/go: invalidate test cache when -coverpkg dependencies change