ci: add max-parallel config to workflows

Author: calebcartwright

Diff for: .github/workflows/integration.yml

@@ -6,6 +6,11 @@ jobs:
     runs-on: ubuntu-latest
     name: ${{ matrix.integration }}
     strategy:
+      # https://help.github.com/en/actions/getting-started-with-github-actions/about-github-actions#usage-limits
+      # There's a limit of 60 concurrent jobs across all repos in the rust-lang organization.
+      # In order to prevent overusing too much of that 60 limit, we throttle the
+      # number of rustfmt jobs that will run concurrently.
+      max-parallel: 4
       fail-fast: false
       matrix:
         integration: [

Diff for: .github/workflows/linux.yml

@@ -6,6 +6,11 @@ jobs:
     runs-on: ubuntu-latest
     name: (${{ matrix.target }}, ${{ matrix.channel }}, ${{ matrix.cfg-release-channel }})
     strategy:
+      # https://help.github.com/en/actions/getting-started-with-github-actions/about-github-actions#usage-limits
+      # There's a limit of 60 concurrent jobs across all repos in the rust-lang organization.
+      # In order to prevent overusing too much of that 60 limit, we throttle the
+      # number of rustfmt jobs that will run concurrently.
+      max-parallel: 1
       fail-fast: false
       matrix:
         target: [

Diff for: .github/workflows/windows.yml

@@ -6,6 +6,11 @@ jobs:
     runs-on: windows-latest
     name: (${{ matrix.target }}, ${{ matrix.channel }})
     strategy:
+      # https://help.github.com/en/actions/getting-started-with-github-actions/about-github-actions#usage-limits
+      # There's a limit of 60 concurrent jobs across all repos in the rust-lang organization.
+      # In order to prevent overusing too much of that 60 limit, we throttle the
+      # number of rustfmt jobs that will run concurrently.
+      max-parallel: 1
       fail-fast: false
       matrix:
         target: [