github: more build file refactoring (#8678)

- Move emscripten download into its own script
 - Refactor the common "CI choice" prompt into its own file.
 - Move the content of `build/common/ci-common.sh` to the
   "CI choice" script.
 - Mention the get-emscripten.sh script in BUILDING.md

.github/workflows/presubmit.yml

@@ -96,6 +96,7 @@ jobs:
         with:
           fetch-depth: 0
       - uses: ./.github/actions/linux-prereq
+      - uses: ./.github/actions/web-prereq
       - name: Run build script
         run: |
           cd build/web && printf "y" | ./build.sh presubmit

.github/workflows/release.yml

@@ -118,6 +118,7 @@ jobs:
         with:
           ref: ${{ steps.git_ref.outputs.ref }}
       - uses: ./.github/actions/linux-prereq
+      - uses: ./.github/actions/web-prereq
       - name: Run build script
         env:
           TAG: ${{ steps.git_ref.outputs.tag }}

.github/workflows/web-continuous.yml

@@ -17,6 +17,7 @@ jobs:
         with:
           fetch-depth: 0
       - uses: ./.github/actions/linux-prereq
+      - uses: ./.github/actions/web-prereq
       - name: Run build script
         run: |
           cd build/web && printf "y" | ./build.sh continuous