fix(ci): use go-test-coverage binary directly to fix Gitea Actions

vladopajic/go-test-coverage@v2 (v2.18.5+) switched composite-action
inputs from CLI args to env-var mapping, which Gitea act_runner does
not expand reliably. The literal string `${{ inputs.config }}` was
passed to the binary, breaking 'Check coverage' steps.

Replace the action with a direct go install + binary invocation
(matching Frostmoln pattern) and use the tool's --github-action-output
to drop manual go tool cover -func calculations.

.testcoverage.yml

@@ -1,7 +1,7 @@
 # Coverage configuration for go-test-coverage
 # https://github.com/vladopajic/go-test-coverage
 
-profile: coverage.txt
+profile: coverage.out
 
 threshold:
   file: 0