[build-script] Add SWIFT_ prefix to USE_SCCACHE env var

Author: porglezomp

docs/DevelopmentTips.md

@@ -36,7 +36,7 @@ $ sccache --start-server
 $ ./swift/utils/build-script MY_ARGS --sccache
 ```
 
-If you want to always use sccache, you can `export USE_SCCACHE=1` and the build script will pick it up.
+If you want to always use sccache, you can `export SWIFT_USE_SCCACHE=1` and the build script will pick it up.
 
 Given the size of artifacts generated, you might also want to bump the cache size from the default 10GB to something larger, say by putting `export SCCACHE_CACHE_SIZE="50G"` in your dotfile(s).
 

utils/build-script

@@ -1077,7 +1077,7 @@ def parse_preset_args():
         help="use sccache",
         action=argparse.actions.StoreTrueAction,
         nargs=argparse.Nargs.OPTIONAL,
-        default=os.environ.get('USE_SCCACHE') == '1')
+        default=os.environ.get('SWIFT_USE_SCCACHE') == '1')
     parser.add_argument(
         "--cmake-c-launcher",
         help="the absolute path to set CMAKE_C_COMPILER_LAUNCHER",

utils/build_swift/build_swift/driver_arguments.py

@@ -388,7 +388,7 @@ def create_argument_parser():
            default=os.environ.get('USE_DISTCC') == '1',
            help='use distcc in pump mode')
     option('--sccache', toggle_true,
-           default=os.environ.get('USE_SCCACHE') == '1',
+           default=os.environ.get('SWIFT_USE_SCCACHE') == '1',
            help='use sccache')
     option('--enable-asan', toggle_true,
            help='enable Address Sanitizer')