Cache dependency downloads on Linux (#566)

In #565 we note
we're seeing rate-limiting on downloads of bzip2 — which is blocking
development here. There, we increase the exponential backoff but we need
to reduce the number of downloads to address the root cause of the
issue. @geofft suggested caching these downloads, which makes a lot of
sense for us — there's already logic to validate the file if it exists
on disk.

I'm just starting with Linux.

Author: zanieb

.github/workflows/linux.yml

@@ -216,6 +216,15 @@ jobs:
           path: build
           merge-multiple: true
 
+      - name: Cache downloads
+        uses: actions/cache@v4
+        with:
+          path: build/downloads
+          key: ${{ matrix.target_triple }}-${{ hashFiles('pythonbuild/downloads.py')}}
+          restore-keys: |
+            ${{ matrix.target_triple }}-${{ hashFiles('pythonbuild/downloads.py')}}
+            ${{ matrix.target_triple }}-
+
       - name: Load Docker Images
         run: |
           for f in build/image-*.tar.zst; do
@@ -307,6 +316,15 @@ jobs:
           path: build
           merge-multiple: true
 
+      - name: Cache downloads
+        uses: actions/cache@v4
+        with:
+          path: build/downloads
+          key: ${{ matrix.target_triple }}-${{ hashFiles('pythonbuild/downloads.py')}}
+          restore-keys: |
+            ${{ matrix.target_triple }}-${{ hashFiles('pythonbuild/downloads.py')}}
+            ${{ matrix.target_triple }}-
+
       - name: Load Docker Images
         run: |
           for f in build/image-*.tar.zst; do