cmd/go: cached artifacts are silently reused if we can't identify the C compiler's version

[bcmills]

Go version

go version devel go1.22-098f059d Mon Dec 4 23:03:04 2023 +0000 linux/amd64

What operating system and processor architecture are you using (go env)?

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/usr/local/google/home/bcmills/.cache/go-build'
GOENV='/usr/local/google/home/bcmills/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/usr/local/google/home/bcmills/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/usr/local/google/home/bcmills'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/google/home/bcmills/go-review'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/usr/local/google/home/bcmills/go-review/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='devel go1.22-ed30ee6c56 Tue Dec 5 21:18:08 2023 +0000'
GCCGO='/usr/bin/gccgo'
GOAMD64='v1'
AR='ar'
CC='/usr/bin/clang'
CXX='c++'
CGO_ENABLED='1'
GOMOD='/usr/local/google/home/bcmills/go-review/src/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build5250197467984633244=/tmp/go-build -gno-record-gcc-switches'

What did you do?

(Forked from #64423.)

1. Configure CC to refer to a C compiler (or wrapper) that doesn't produce a recognizable version string when run as $CC -### -x c -c -, but works otherwise.
2. Run go test cmd/go -run=TestScript/cgo_stale_precompiled.

What did you expect to see?

Either the entire test passes (because some other property of the compiler is used as its cache ID instead), or builds that depend on the output artifacts of the C compiler fail with an informative error message.

What did you see instead?

Per #64423, go build commands succeed but the cache fails to distinguish between “no usable C compiler” and “the C compiler is unchanged”: removing the compiler from $PATH erroneously fails to invalidate the cache.

(I am fixing #64423 by fixing the version-parsing logic, but the error-handling bug remains.)