integration tests: Allow literal '$' in expected contents

Escape a literal $ with $$.

internal/tshelpers/custom_commands.go

@@ -397,6 +397,9 @@ func archiveFileNames(isoPath string) (string, string, error) {
 
 func expand(ts *testscript.TestScript, s []byte) string {
 	return os.Expand(string(s), func(key string) string {
+		if key == "$" {
+			return "$"
+		}
 		return ts.Getenv(key)
 	})
 }