Prevent a the new workflows from running concurrently

The newly-added workflows update global state. This global state is held
in Git notes in the `gitgitgadget/git` repository and is updated in
reaction to updates to the Git mailing list mirror and to the `seen`
branch.

While there is now logic in GitGitGadget's Git notes handling to allow
for "distributed updates", it makes more sense to hold off from trying
to perform (nearly-)identical updates in parallel.

Note: In the case of `handle-pr-comment` and `handle-pr-push`, let's be
mindful that it should be possible to handle different PRs concurrently,
which is definitely a change of behavior from the current Azure
Pipelines setup, where only one job can be run at any given time,
_across_ Azure Pipelines (for example, a lengthy `Update GitGitGadget's
PRs` run can easily delay any `/submit` from being processed).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/handle-new-mails.yml

@@ -4,6 +4,9 @@ on:
   # GitGitGadget's GitHub App is expected to trigger this on `git-mailing-list-mirror` updates
   workflow_dispatch:
 
+concurrency:
+  group: ${{ github.workflow }}
+
 jobs:
   handle-new-mails:
     runs-on: ubuntu-latest

.github/workflows/handle-pr-comment.yml

@@ -13,6 +13,9 @@ on:
 env:
   PR_COMMENT_URL: ${{ inputs.pr-comment-url }}
 
+concurrency:
+  group: ${{ github.workflow }}-${{ inputs.pr-comment-url }}
+
 jobs:
   handle-pr-comment:
     runs-on: ubuntu-latest

.github/workflows/handle-pr-push.yml

@@ -13,6 +13,9 @@ on:
 env:
   PR_URL: ${{ inputs.pr-url }}
 
+concurrency:
+  group: ${{ github.workflow }}-${{ inputs.pr-url }}
+
 jobs:
   handle-pr-push:
     runs-on: ubuntu-latest

.github/workflows/update-mail-to-commit-notes.yml

@@ -5,6 +5,9 @@ on:
   # GitGitGadget's GitHub App is expected to trigger this on `seen` updates
   workflow_dispatch:
 
+concurrency:
+  group: ${{ github.workflow }}
+
 jobs:
   update-mail-to-commit-notes:
     runs-on: ubuntu-latest

.github/workflows/update-prs.yml

@@ -4,6 +4,9 @@ on:
   # GitGitGadget's GitHub App is expected to trigger this on `seen` updates
   workflow_dispatch:
 
+concurrency:
+  group: ${{ github.workflow }}
+
 jobs:
   update-prs:
     runs-on: ubuntu-latest