ci: ignore Cachix errors (#952)

This commit allows CI jobs to degrade gracefully if Cachix has issues,
by skipping uploads and/or building packages from scratch rather than
failing the job.

This is in response to the recent failures on `master`, which appear to
be caused by a 502 (Bad Gateway) response while uploading some of the
build results.

.github/workflows/check.yml

@@ -22,6 +22,7 @@ jobs:
         with:
           name: stylix
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+        continue-on-error: true
 
       - id: get-derivations
         run: |
@@ -93,6 +94,7 @@ jobs:
         with:
           name: stylix
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+        continue-on-error: true
 
       - run: |
           nix build --no-update-lock-file --print-build-logs \

.github/workflows/docs.yml

@@ -32,6 +32,7 @@ jobs:
         with:
           name: stylix
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+        continue-on-error: true
 
       - run: nix build .#docs