ci: force Pkg.build for docs env in Downgrade workflow (#96)

The downgrade-compat resolver half-installs the minimal-version docs
manifest (e.g. legacy GR without GR_jll) without running build
scripts, so libGR.so never gets downloaded and docdeploy fails.
Explicitly building the docs project closes that gap.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sethaxen

.github/workflows/Downgrade.yml

@@ -62,6 +62,7 @@ jobs:
           projects: ., docs
           julia_version: ${{ steps['setup-julia'].outputs.julia-version }}
       - uses: julia-actions/julia-buildpkg@v1
+      - run: julia --project=docs -e 'using Pkg; Pkg.build()'
       - uses: julia-actions/julia-docdeploy@v1
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}