ci: fix dependabot duplicated workflow runs

Every workflow is being executed twice for dependabot: Once when its
branch is pushed to this repository and again when a PR is opened for
it.

For example, see the checks in #5979 ("29 checks passed").

This happens because both `on.push` and `on.pull_request` are specified
in the workflow files.

There does not seem to be a simple and generic way to avoid such
duplicated runs directly in GitHub Actions (such as preventing the same
check from running for the same exact commit)[1], so just ignore the
dependabot branches on push for now.

See also and commit 5871b08a4 ("ci: run for every branch instead of just
master", 2023-04-23) / PR #5815.

[1] https://github.com/orgs/community/discussions/26276

1 files changed, 2 insertions, 0 deletions

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index bd6fdb2e1..607c2e59c 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -6,6 +6,8 @@ name: Build
 # Note: Keep this list in sync with DISTFILES in ../../Makefile.
 on:
   push:
+    branches-ignore:
+      - 'dependabot/**'
     paths:
       - 'contrib/**'
       - 'etc/**'