ci: set CI_JOB_IMAGE for coverity job

The main GitHub Actions workflow switched away from the "$distro"
variable in b133d30 (github: simplify computation of the job's
distro, 2025-01-10). Since the Coverity job also depends on our
ci/install-dependencies.sh script, it needs to likewise set CI_JOB_IMAGE
to find the correct dependencies (without this patch, we don't install
curl and the build fails).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

.github/workflows/coverity.yml

@@ -45,7 +45,7 @@ jobs:
       - run: ci/install-dependencies.sh
         if: contains(matrix.os, 'ubuntu') || contains(matrix.os, 'macos')
         env:
-          distro: ${{ matrix.os }}
+          CI_JOB_IMAGE: ${{ matrix.os }}
 
       # The Coverity site says the tool is usually updated twice yearly, so the
       # MD5 of download can be used to determine whether there's been an update.