reject invalid Go toolchain versions early

Otherwise we might do odd things, such as trying to open the linker
patches directory "patches/" given that go/version.Lang
returns an empty string for invalid versions.

See #978.

main.go

@@ -546,6 +546,10 @@ func goVersionOK() bool {
 		return false
 	}
 
+	if !version.IsValid(toolchainVersion) {
+		fmt.Fprintf(os.Stderr, "Go version %q appears to be invalid or too old; use %s or newer\n", toolchainVersion, minGoVersion)
+		return false
+	}
 	if version.Compare(toolchainVersion, minGoVersion) < 0 {
 		fmt.Fprintf(os.Stderr, "Go version %q is too old; please upgrade to %s or newer\n", toolchainVersion, minGoVersion)
 		return false

testdata/script/goversion.txtar

@@ -9,15 +9,20 @@ env TOOLCHAIN_GOVERSION=''
 ! exec garble build
 stderr 'Go version is too old; please upgrade to go1\.25\.0 or newer'
 
+# A clearly invalid go version.
+env TOOLCHAIN_GOVERSION='bogus version'
+! exec garble build
+stderr 'Go version "bogus version" appears to be invalid or too old; use go1\.25\.0 or newer'
+
 # We should error on a devel version that's too old;
 # note that they started with the string "devel",
 # and very old ones didn't even have "goN.M" in them.
 env TOOLCHAIN_GOVERSION='devel +afb5fca Sun Aug 07 00:00:00 2020 +0000'
 ! exec garble build
-stderr 'Go version "devel \+afb5.*2020.*" is too old; please upgrade to go1\.25\.0 or newer'
+stderr 'Go version .* appears to be invalid or too old'
 env TOOLCHAIN_GOVERSION='devel go1.15-afb5fca Sun Aug 07 00:00:00 2020 +0000'
 ! exec garble build
-stderr 'Go version "devel go1\.15-.*2020.*" is too old; please upgrade to go1\.25\.0 or newer'
+stderr 'Go version .* appears to be invalid or too old'
 
 # A current devel version should be fine.
 # Note that we don't look at devel version timestamps.