We skip many gccgo tests by setting GCCGO to a bogus value. However,
this particular test doesn't properly handle a non-existing $GCCGO
hence we unset the environment variable here.

diff -upr go.orig/src/cmd/go/testdata/script/gccgo_link_c.txt go/src/cmd/go/testdata/script/gccgo_link_c.txt
--- go.orig/src/cmd/go/testdata/script/gccgo_link_c.txt	2022-09-30 10:58:07.524516646 +0200
+++ go/src/cmd/go/testdata/script/gccgo_link_c.txt	2022-09-30 10:58:29.441202925 +0200
@@ -4,6 +4,8 @@
 [!cgo] skip
 [!exec:gccgo] skip
 
+env GCCGO=
+
 go build -n -compiler gccgo
 stderr 'gccgo.*\-L [^ ]*alibpath \-lalib' # make sure that Go-inline "#cgo LDFLAGS:" ("-L alibpath -lalib") passed to gccgo linking stage