pin Xcode 11.7 as the new default 12.0.1 is broken

Xcode has been changed to default to Xcode 12.0.1
(actions/runner-images#1712), but this
breaks the build. This change pins builds to Xcode 11.7 until it can be
fixed.

.github/workflows/cpp.yml

@@ -207,6 +207,9 @@ jobs:
       ARROW_WITH_SNAPPY: ON
       ARROW_WITH_ZLIB: ON
       ARROW_WITH_ZSTD: ON
+      # Pin Xcode to 11.7 instead of the new default of 12.0.1
+      # See: https://github.com/actions/virtual-environments/issues/1712
+      DEVELOPER_DIR: /Applications/Xcode_11.7.app/Contents/Developer
     steps:
       - name: Checkout Arrow
         uses: actions/checkout@v2

.github/workflows/python.yml

@@ -136,6 +136,9 @@ jobs:
       ARROW_WITH_BROTLI: ON
       ARROW_BUILD_TESTS: OFF
       CMAKE_ARGS: "-DPYTHON_EXECUTABLE=/usr/local/bin/python3"
+      # Pin Xcode to 11.7 instead of the new default of 12.0.1
+      # See: https://github.com/actions/virtual-environments/issues/1712
+      DEVELOPER_DIR: /Applications/Xcode_11.7.app/Contents/Developer
     steps:
       - name: Checkout Arrow
         uses: actions/checkout@v2

.github/workflows/ruby.yml

@@ -118,6 +118,9 @@ jobs:
       ARROW_WITH_SNAPPY: ON
       ARROW_WITH_ZLIB: ON
       ARROW_WITH_ZSTD: ON
+      # Pin Xcode to 11.7 instead of the new default of 12.0.1
+      # See: https://github.com/actions/virtual-environments/issues/1712
+      DEVELOPER_DIR: /Applications/Xcode_11.7.app/Contents/Developer
       XML_CATALOG_FILES: /usr/local/etc/xml/catalog
     steps:
       - name: Checkout Arrow