github: Set a default for the "branch" input for manual workflows

.github/workflows/docker.yml

@@ -8,6 +8,7 @@ on:
       branch:
         description: 'Branch to extract from'
         type: string
+        default: 'master'
       login:
         description: 'Log in to Docker Hub'
         type: boolean

.github/workflows/msvcrt.yml

@@ -8,6 +8,7 @@ on:
       branch:
         description: 'Branch to extract from'
         type: string
+        default: 'master'
       upload:
         description: 'Upload the artifacts'
         type: boolean

.github/workflows/release.yml

@@ -8,6 +8,7 @@ on:
       branch:
         description: 'Branch to extract from'
         type: string
+        default: 'master'
       tag:
         description: 'Tag to create/upload to'
         required: true

.github/workflows/test-libcxx.yml

@@ -8,6 +8,7 @@ on:
       branch:
         description: 'Branch to extract from'
         type: string
+        default: 'master'
       pipeline_type:
         description: 'Pipeline type'
         required: true

.github/workflows/test-llvm.yml

@@ -8,6 +8,7 @@ on:
       branch:
         description: 'Branch to extract from'
         type: string
+        default: 'master'
       pipeline_type:
         description: 'Pipeline type'
         required: true