CI: Travis: add: cache: for /nix/store, would share between master and inside PRs

M  .travis.yml

.travis.yml

@@ -147,6 +147,12 @@ script:
   #
   #
 
+# NOTE: Travis cache allows near fast cache loading of Nix pkgs store,
+# Since the Cachix does not cache PRs - Travis cache is great because it caches PR branch builds
+# During devepment of this PR I had ~40GB cache on the PR branch.
+cache:
+  directories:
+    - /nix/store
 branches:
   only:
     - master