do not install Linux packages if build is not needed

Author: kinkie

.github/workflows/gen-docs.yaml

@@ -14,15 +14,6 @@ jobs:
     runs-on: ubuntu-24.04
 
     steps:
-      - name: Install prerequisite Linux packages
-        run: |
-          # required for "apt-get build-dep" to work
-          sudo sed --in-place -E 's/^(Types: deb)$/\1 deb-src/' /etc/apt/sources.list.d/ubuntu.sources
-          sudo apt-get --quiet=2 update
-          sudo apt-get --quiet=2 build-dep squid
-          sudo apt-get --quiet=2 install libtool-bin doxygen graphviz pdf2svg
-
-
       - name: Checkout Docs
         uses: actions/checkout@v5
         with:
@@ -56,6 +47,15 @@ jobs:
           fi
           echo $CURRENT_COMMIT_HASH > ../last-squid-commit.txt
 
+      - name: Install prerequisite Linux packages
+        if: steps.check_squid_updates.outputs.had_update == 'true'
+        run: |
+          # required for "apt-get build-dep" to work
+          sudo sed --in-place -E 's/^(Types: deb)$/\1 deb-src/' /etc/apt/sources.list.d/ubuntu.sources
+          sudo apt-get --quiet=2 update
+          sudo apt-get --quiet=2 build-dep squid
+          sudo apt-get --quiet=2 install libtool-bin doxygen graphviz pdf2svg
+
       - name: Prepare squid sources
         working-directory: squid
         if: steps.check_squid_updates.outputs.had_update == 'true'