ci: Split cabal and stack work caches from regular cache

This is a first step to split up the cabal and stack caches in separate
pieces. Here we split the work folder, which just contains the
postgrest-specific build artifacts, into a separate cache.

More fine-grained caching should give us better cache hits and much
fewer upload size in the regular case, improving CI performance.

Since the work file caches are very small (about 30-40 MB) they are
cached for PRs, too. This will allow the majority of PRs, which only
change source code files, but no dependencies, to still have cached
their build files for additional commits.

.cirrus.yml

@@ -19,8 +19,20 @@ build_task:
 
   stack_cache:
     folders: /.stack
-    fingerprint_script: cat postgrest.cabal stack.yaml.lock
-    reupload_on_changes: false
+    fingerprint_script:
+      - echo $CIRRUS_OS
+      - stack --version
+      - md5sum postgrest.cabal
+      - md5sum stack.yaml.lock
+
+  stack_work_cache:
+    folders: .stack-work
+    fingerprint_script:
+      - echo $CIRRUS_OS
+      - stack --version
+      - md5sum postgrest.cabal
+      - md5sum stack.yaml.lock
+      - find main src -type f -iname '*.hs' -exec md5sum "{}" +
 
   build_script: |
     stack build -j 1 --local-bin-path . --copy-bins --stack-yaml stack-21.7.yaml