ci: use bash when executing the "bors build finished" jobs

We don't clone the repository in those builders, so the default shell
(src/ci/exec-with-shell.py) is not present there.

.github/workflows/ci.yml

@@ -624,6 +624,7 @@ jobs:
     steps:
       - name: mark the job as a success
         run: exit 0
+        shell: bash
     name: bors build finished
     runs-on: ubuntu-latest
   try-failure:
@@ -633,6 +634,7 @@ jobs:
     steps:
       - name: mark the job as a failure
         run: exit 1
+        shell: bash
     name: bors build finished
     runs-on: ubuntu-latest
   auto-success:
@@ -642,6 +644,7 @@ jobs:
     steps:
       - name: mark the job as a success
         run: exit 0
+        shell: bash
     name: bors build finished
     runs-on: ubuntu-latest
   auto-failure:
@@ -651,5 +654,6 @@ jobs:
     steps:
       - name: mark the job as a failure
         run: exit 1
+        shell: bash
     name: bors build finished
     runs-on: ubuntu-latest

src/ci/github-actions/ci.yml

@@ -210,12 +210,14 @@ x--expand-yaml-anchors--remove:
     steps:
       - name: mark the job as a success
         run: exit 0
+        shell: bash
     <<: *base-outcome-job
 
   - &base-failure-job
     steps:
       - name: mark the job as a failure
         run: exit 1
+        shell: bash
     <<: *base-outcome-job
 
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