CI: Cache dependencies

Attempt to cache dependencies with the travis cache. This avoids
redownloading avr-gcc for each stage of the pipeline.

This /might/ improve the download failures we're seeing recently,
although the travis cache is also networked.

.travis.yml

@@ -1,6 +1,10 @@
 dist: focal
 language: minimal
 
+cache:
+  directories:
+    .dependencies/
+
 before_install:
   # Prepare the dependencies for the old build environment
   - sudo apt-get install -y python3-polib python3-pyelftools python3-regex