cmd/go: test -cover can't find covdata tool with switched toolchain and empty tests [1.26 backport]

[gopherbot]

@matloob requested issue #75031 to be considered for backport to the next 1.26 minor release.

@gopherbot please backport

[gopherbot]

Change https://go.dev/cl/760500 mentions this issue: [release-branch.go1.26] cmd/go: specify full path to go command when running go tool covdata