[ci] Skip GitHub workflow if changes don't affect the build

The GitHub workflow runs even if the changes don't affect the Linux
build.

Skip workflow runs for commits that don't affect the Linux build.

Negative path patterns are used in instead of paths-ignore: if anything
in paths-ignore matches, the workflow will be skipped, which is too
aggressive. Instead, the workflow should be skipped if the change solely
affects ignored files.

.github/workflows/linux.yml

@@ -1,11 +1,43 @@
 ---
 name: Linux CI
 
+# We want to skip workflow runs for commits that don't affect the build. We
+# use negative path patterns instead of paths-ignore: if anything in
+# paths-ignore matches, the workflow will be skipped, which is too
+# aggressive. Instead, we want to skip if the commit(s) solely affects
+# ignored files.
+#
+# See
+# https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#example-using-positive-and-negative-patterns-1
 on:
   push:
     branches: ["master"]
+    paths:
+      # Include everything by default
+      - '**'
+      # CHANGELOG.md is high-churn, but don't affect the build.
+      #
+      # NB: README.md is NOT in this list; the documentation build consumes
+      # it.
+      - '!CHANGELOG.md'
+      # These files are only used by the Windows CI builds and don't affect
+      # the Linux builds.
+      - '!appveyor.yml'
+      - '!tools/ci-scripts/windows/**'
   pull_request:
     branches: ["master"]
+    paths:
+      # Include everything by default
+      - '**'
+      # CHANGELOG.md is high-churn, but don't affect the build.
+      #
+      # NB: README.md is NOT in this list; the documentation build consumes
+      # it.
+      - '!CHANGELOG.md'
+      # These files are only used by the Windows CI builds and don't affect
+      # the Linux builds.
+      - '!appveyor.yml'
+      - '!tools/ci-scripts/windows/**'
 
 jobs:
   tests: