cmd/go: crash on unknown GOEXPERIMENT during toolchain selection [1.24 backport]

[gopherbot]

@prattmic requested issue #74111 to be considered for backport to the next 1.24 minor release.

@gopherbot Please backport to 1.24. This is a regression in passing GOEXPERIMENT to another version selected via GOTOOLCHAIN.

[gopherbot]

Change https://go.dev/cl/682735 mentions this issue: [release-branch.go1.24] cmd/go/internal/fips140: ignore GOEXPERIMENT on error