Merge #16214: travis: Fix caching issues

88884c6f75 travis: Use absolute paths for cache dirs (MarcoFalke)
fae9d54abc travis: Fix caching issues (MarcoFalke)

Pull request description:

  It appears that the travis caching infrastructure changed under us without any notice. I believe we can no longer use relative paths as cache paths, unless:
  * we go back to the root travis build dir before the caching step, or
  * we specify absolute paths for caching

  Apply both fixes here.

  Thanks to **promag** for helping me debug this: https://github.com/bitcoin/bitcoin/issues/16148#issuecomment-502078938

ACKs for commit 88884c:
  Empact:
    ACK 88884c6f75

Tree-SHA512: 04f2987aade4e8bb016862ba81aea4bb90573a0bf0d2e51b0411c6e3687ee8ec3b639627c0950f51bc8ae4bbf5e0799672c9a81dfb03f01eb5b08791ba857a4a

.travis.yml

@@ -31,8 +31,8 @@ language: minimal
 cache:
   ccache: true
   directories:
-    - depends/built
-    - depends/sdk-sources
+    - $TRAVIS_BUILD_DIR/depends/built
+    - $TRAVIS_BUILD_DIR/depends/sdk-sources
     - $HOME/.ccache
 stages:
   - lint

.travis/test_06_script_b.sh

@@ -25,3 +25,5 @@ if [ "$RUN_FUZZ_TESTS" = "true" ]; then
   DOCKER_EXEC test/fuzz/test_runner.py -l DEBUG ${DIR_FUZZ_IN}
   END_FOLD
 fi
+
+cd ${TRAVIS_BUILD_DIR} || (echo "could not enter travis build dir $TRAVIS_BUILD_DIR"; exit 1)