Restrict publishing job to the main repository. (#104)

See the
[documentation](https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idif)
for details on the syntax, and the [context
docs](https://docs.github.com/en/actions/reference/context-and-expression-syntax-for-github-actions#github-context)
for the API used.

.github/workflows/publish-docs.yaml

@@ -6,6 +6,7 @@ on:
 
 jobs:
   build:
+    if: github.repository == 'carbon-language/carbon-lang'
     runs-on: [ubuntu-latest]
     steps:
       - uses: actions/checkout@v2