Remove unnecessary GitHub Action triggers

This goes from `[push, pull_request]` to just `push` on all four GitHub Actions.

I also added `paths` elements to each GitHub Action that will hopefully reduce the amount of unnecessary work in GitHub Actions.

I also added `on: workflow_dispatch` so we should be able to manually trigger each of these actions if necessary.

Author: NicMcPhee

.github/workflows/arraymerge_gtest.yml

@@ -1,8 +1,17 @@
 name: array-merge-gtest
 
-on: [push, pull_request]
-#    paths:
-#    - 'array_merge/**'
+on: push
+  paths:
+    - 'array_merge/**'
+    # We probably want to re-run the checks if the definition of
+    # `mergesort` changes since this depends on that.
+    - 'mergesort/mergesort.[ch]'
+
+# This lets us manually trigger this action
+# through the GitHub web interface. See
+# https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_dispatch
+# for more details.
+on: workflow_dispatch
 
 jobs:
   build:

.github/workflows/arraymerge_test_valgrind.yml

@@ -1,8 +1,17 @@
 name: array-merge-test-valgrind
 
-on: [push, pull_request]
-#    paths:
-#    - 'array_merge/**'
+on: push
+  paths:
+    - 'array_merge/**'
+    # We probably want to re-run the checks if the definition of
+    # `mergesort` changes since this depends on that.
+    - 'mergesort/mergesort.[ch]'
+
+# This lets us manually trigger this action
+# through the GitHub web interface. See
+# https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_dispatch
+# for more details.
+on: workflow_dispatch
 
 jobs:
   build:

.github/workflows/mergesort_gtest.yml

@@ -1,8 +1,14 @@
 name: mergesort-gtest
 
-on: [push, pull_request]
-#    paths:
-#    - 'mergesort/**'
+on: push
+  paths:
+    - 'mergesort/**'
+
+# This lets us manually trigger this action
+# through the GitHub web interface. See
+# https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_dispatch
+# for more details.
+on: workflow_dispatch
 
 jobs:
   build:

.github/workflows/mergesort_test_valgrind.yml

@@ -1,8 +1,14 @@
 name: mergesort-test-valgrind
 
-on: [push, pull_request]
-#    paths:
-#    - 'mergesort/**'
+on: push
+  paths:
+    - 'mergesort/**'
+
+# This lets us manually trigger this action
+# through the GitHub web interface. See
+# https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_dispatch
+# for more details.
+on: workflow_dispatch
 
 jobs:
   build: