Prevent actions from running on draft PRs (#2457)

Draft PRs are triggering actions on every commit (except
`test-invoke-pip.yml`).

I've added a conditional to each job to only run when the PR is not a
draft.

(maybe there is a reason we are running all applicable workflows on
draft PRs?)

.github/workflows/build-cloud-img.yml

@@ -21,6 +21,7 @@ env:
 
 jobs:
   docker:
+    if: github.event.pull_request.draft == false
     strategy:
       fail-fast: false
       matrix:

.github/workflows/build-container.yml

@@ -8,6 +8,7 @@ on:
 
 jobs:
   docker:
+    if: github.event.pull_request.draft == false
     strategy:
       fail-fast: false
       matrix:

.github/workflows/lint-frontend.yml

@@ -14,6 +14,7 @@ defaults:
 
 jobs:
   lint-frontend:
+    if: github.event.pull_request.draft == false
     runs-on: ubuntu-22.04
     steps:
       - name: Setup Node 18

.github/workflows/mkdocs-material.yml

@@ -7,6 +7,7 @@ on:
 
 jobs:
   mkdocs-material:
+    if: github.event.pull_request.draft == false
     runs-on: ubuntu-latest
     steps:
       - name: checkout sources

.github/workflows/pyflakes.yml

@@ -9,6 +9,7 @@ on:
 jobs:
   pyflakes:
     name: runner / pyflakes
+    if: github.event.pull_request.draft == false
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2