Set GOROOT

GOROOT was dropped by actions/setup-go#175.

If it is not set then this test fails with:

    --- FAIL: TestGVMRunUse (5.45s)
    --- FAIL: TestGVMRunUse/1.7.4_bash (2.68s)
        use_test.go:54: export GOROOT="/home/runner/.gvm/versions/go1.7.4.linux.amd64"
            export PATH="/home/runner/.gvm/versions/go1.7.4.linux.amd64/bin:$PATH"

        use_test.go:79: failed to run go version: exit status 2
            go: cannot find GOROOT directory: /usr/local/go

cmd/gvm/use_test.go

@@ -72,7 +72,10 @@ func TestGVMRunUse(t *testing.T) {
 			}
 
 			// Test that GOROOT/bin/go exists and is the correct version.
-			version, err := exec.Command(filepath.Join(goroot, "bin", "go"), "version").Output()
+			goVersionCmd := exec.Command(filepath.Join(goroot, "bin", "go"), "version")
+			goVersionCmd.Env = append(goVersionCmd.Env, "GOROOT="+goroot)
+
+			version, err := goVersionCmd.Output()
 			if err != nil {
 				var exitErr *exec.ExitError
 				if errors.As(err, &exitErr) {