.cirrus.yml: add golangci-lint cache for freebsd task

This shaves the linting time:
 - from 8 to 2 minutes on Cirrus CI with 4 CPUs and 8 GB RAM;
 - from 11 to 3 minutes on Cirrus CI with 2 CPUs and 4 GB RAM;
when the cache is reused.

The cache size is quite modest (~3MB compressed, ~7MB uncompressed).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.cirrus.yml

@@ -333,6 +333,13 @@ freebsd_alt_build_task:
         memory: 8Gb
     setup_script:
         - pkg install -y gpgme bash go-md2man gmake gsed gnugrep go pkgconf zstd
+        - go version # Downloads a new go version based on go.mod's go directive.
+    golint_cache:
+        folder: ~/.cache/golangci-lint
+        reupload_on_changes: true
+        fingerprint_script:
+            - go version
+            - grep GOLANGCI_LINT_VERSION Makefile | head -1
     lint_script:
         - gmake golangci-lint
     build_amd64_script: