add cancel in progress support to automated CI checks (#11130)

# Objective

When working on PRs, I'll often find that one of the early CI checks
fails, and work on fixing the result, but when I push the earlier
commits are still being processed by the CI. This would mean that if a
new commit is pushed while another CI process is already running on that
branch, the first set of jobs will be cancelled - reducing wasted
resources and wait time for CI on the latest commits.

## Solution

The solution is simply adding Github's concurrency groups to every
relevant workflow.

.github/workflows/ci.yml

@@ -11,6 +11,10 @@ env:
   CARGO_TERM_COLOR: always
   NIGHTLY_TOOLCHAIN: nightly
 
+concurrency:
+  group: ${{github.workflow}}-${{github.ref}}
+  cancel-in-progress: ${{github.event_name == 'pull_request'}}
+
 jobs:
   build:
     strategy:

.github/workflows/dependencies.yml

@@ -12,6 +12,10 @@ on:
     branches:
       - main
 
+concurrency:
+  group: ${{github.workflow}}-${{github.ref}}
+  cancel-in-progress: ${{github.event_name == 'pull_request'}}
+
 env:
   CARGO_TERM_COLOR: always
 

.github/workflows/docs.yml

@@ -4,7 +4,6 @@ on:
   push:
     branches:
       - 'main'
-
   # Allows running the action manually.
   workflow_dispatch:
 

.github/workflows/validation-jobs.yml

@@ -7,6 +7,11 @@ on:
     branches:
       - main
 
+
+concurrency:
+  group: ${{github.workflow}}-${{github.ref}}
+  cancel-in-progress: ${{github.event_name == 'pull_request'}}
+
 env:
   CARGO_TERM_COLOR: always
   NIGHTLY_TOOLCHAIN: nightly