cmd/go: issue33139 is only valid when running with CGO

With CGO disabled, the test suite tries to run the following and fail:

CGO_ENABLED=0 go test -run=TestScript/link_syso_issue33139 cmd/go
go test proxy running at GOPROXY=http://127.0.0.1:38829/mod
--- FAIL: TestScript (0.01s)
    --- FAIL: TestScript/link_syso_issue33139 (0.01s)
        script_test.go:215:
            # Test that we can use the external linker with a host syso file that is
            # embedded in a package, that is referenced by a Go assembly function.
            # See issue 33139. (0.000s)
            # External linking is not supported on linux/ppc64.
            # See: #8912 (0.000s)
            # External linking is not supported on linux/riscv64.
            # See: #36739 (0.001s)
            > [linux] [riscv64] skip
            > cc -c -o syso/objTestImpl.syso syso/src/objTestImpl.c
            FAIL: testdata/script/link_syso_issue33139.txt:15:
                unexpected error starting command:
                        fork/exec /dev/null: permission denied

The existing filter [!exec:cc] skip is not sufficient in the case when
CC is deliberately set to a file not meant to be executed while building
the non cgo variant of Go

src/cmd/go/testdata/script/link_syso_issue33139.txt

@@ -3,6 +3,7 @@
 # See issue 33139.
 [!gc] skip
 [!exec:cc] skip
+[!cgo] skip
 
 # External linking is not supported on linux/ppc64.
 # See: https://github.com/golang/go/issues/8912