fix: ci: Drop ccache support from CI

The population of the compile cache is so slow, that a BIND 9 build can
take 20 minutes.

Assisted-by: Claude:claude-opus-4-7

Merge branch 'mnowak/drop-ccache-support' into 'main'

See merge request isc-projects/bind9!12127

.gitlab-ci.yml

@@ -17,7 +17,6 @@ variables:
   GIT_COMMITTER_EMAIL: "$GIT_AUTHOR_EMAIL"
 
   CI_REGISTRY_IMAGE: registry.gitlab.isc.org/isc-projects/images/bind9
-  CCACHE_DIR: "/ccache"
 
   GIT_DEPTH: 1
   GIT_CLEAN_FLAGS: -ffdxq
@@ -464,7 +463,6 @@ stages:
   <<: *default_triggering_rules
   stage: build
   before_script:
-    - test -w "${CCACHE_DIR}" && export PATH="/usr/lib/ccache:${PATH}"
     - *list_installed_package_versions
   script:
     - *configure