Revert "Replace set-output as its deprecated by github (#315)"

This reverts commit faa376f.

This change ([release 1.2.8][1]) broke the output functionality
unfortunately, so reverting it.  Will look into the cause in due course
(it's not urgent, as the [`set-output` functionality will not be
deprecated until 31st May 2023][2]).

[1]: https://github.com/agilepathway/label-checker/releases/tag/v1.2.8
[2]: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

internal/github/action.go

@@ -106,7 +106,7 @@ func (a *Action) pullRequestNumber() int {
 }
 
 func (a *Action) resultStepOutput(result string) string {
-	return fmt.Sprintf("echo 'label_check=%s' >> $GITHUB_OUTPUT", result)
+	return "::set-output name=label_check::" + result
 }
 
 func (a *Action) token() string {

label_checker_test.go

@@ -115,10 +115,10 @@ func TestLabelChecks(t *testing.T) {
 		t.Run(name, func(t *testing.T) {
 			tc.expectedStdout = "Checking GitHub labels ...\n" + tc.expectedStdout
 			if len(tc.expectedStderr) > 0 {
-				tc.expectedStdout += "echo 'label_check=failure' >> $GITHUB_OUTPUT\n"
+				tc.expectedStdout += "::set-output name=label_check::failure\n"
 				tc.expectedStderr = "Error: " + tc.expectedStderr
 			} else {
-				tc.expectedStdout += "echo 'label_check=success' >> $GITHUB_OUTPUT\n"
+				tc.expectedStdout += "::set-output name=label_check::success\n"
 			}
 			setPullRequestNumber(tc.prNumber)
 			tc.specifyChecks()
@@ -191,10 +191,10 @@ func TestLabelChecksWithFailureAllowed(t *testing.T) {
 		t.Run(name, func(t *testing.T) {
 			tc.expectedStdout = "Checking GitHub labels ...\n" + tc.expectedStdout
 			if len(tc.expectedStderr) > 0 {
-				tc.expectedStdout += "echo 'label_check=failure' >> $GITHUB_OUTPUT\n"
+				tc.expectedStdout += "::set-output name=label_check::failure\n"
 				tc.expectedStderr = "Error: " + tc.expectedStderr
 			} else {
-				tc.expectedStdout += "echo 'label_check=success' >> $GITHUB_OUTPUT\n"
+				tc.expectedStdout += "::set-output name=label_check::success\n"
 			}
 			setPullRequestNumber(tc.prNumber)
 			tc.specifyChecks()