ci: Move linter task to cirrus

ci/lint/06_script.sh

@@ -6,11 +6,11 @@
 
 export LC_ALL=C
 
-if [ "$TRAVIS_EVENT_TYPE" = "pull_request" ]; then
-  # TRAVIS_BRANCH will be present in a Travis environment. For builds triggered
+if [ -n "$CIRRUS_PR" ]; then
+  # CIRRUS_PR will be present in a Cirrus environment. For builds triggered
   # by a pull request this is the name of the branch targeted by the pull request.
-  # https://docs.travis-ci.com/user/environment-variables/
-  COMMIT_RANGE="$TRAVIS_BRANCH..HEAD"
+  # https://cirrus-ci.org/guide/writing-tasks/#environment-variables
+  COMMIT_RANGE="$CIRRUS_BRANCH..HEAD"
   test/lint/commit-script-check.sh $COMMIT_RANGE
 fi
 
@@ -25,8 +25,8 @@ test/lint/check-doc.py
 test/lint/check-rpc-mappings.py .
 test/lint/lint-all.sh
 
-if [ "$TRAVIS_REPO_SLUG" = "bitcoin/bitcoin" ] && [ "$TRAVIS_EVENT_TYPE" = "cron" ]; then
+if [ "$CIRRUS_REPO_FULL_NAME" = "bitcoin/bitcoin" ] && [ -n "$CIRRUS_CRON" ]; then
     git log --merges --before="2 days ago" -1 --format='%H' > ./contrib/verify-commits/trusted-sha512-root-commit
-    travis_retry gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys $(<contrib/verify-commits/trusted-keys) &&
+    ${CI_RETRY_EXE}  gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys $(<contrib/verify-commits/trusted-keys) &&
     ./contrib/verify-commits/verify-commits.py --clean-merge=2;
 fi