ci: update coverity runs_on_pool reference

Commit 2d65e5b6a6 (ci: rename "runs_on_pool" to "distro", 2024-04-12)
renamed this variable for the main CI workflow, as well as in the ci/
scripts. Because the coverity workflow also relies on those scripts to
install dependencies, it needs to be updated, too. Without this patch,
the coverity build fails because we lack libcurl.

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

.github/workflows/coverity.yml

@@ -45,7 +45,7 @@ jobs:
       - run: ci/install-dependencies.sh
         if: contains(matrix.os, 'ubuntu') || contains(matrix.os, 'macos')
         env:
-          runs_on_pool: ${{ matrix.os }}
+          distro: ${{ 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.