fbcode_builder: fixup expansion of CC/CXX

Summary:
These were failing in the watchman build

```
$ cd build && python fbcode_builder/shell_builder.py > ~/run.sh && bash ~/run.sh
Traceback (most recent call last):
  File "fbcode_builder/shell_builder.py", line 110, in <module>
    steps = make_steps(builder)
  File "/home/travis/build/facebook/watchman/build/fbcode_builder/utils.py", line 94, in <lambda>
    steps_for_spec(builder, config['fbcode_builder_spec'](builder))
  File "/home/travis/build/facebook/watchman/build/fbcode_builder/fbcode_builder.py", line 144, in build
    return [self.setup(), self.diagnostics()] + steps
  File "fbcode_builder/shell_builder.py", line 61, in setup
    ).format(ccache_dir=ccache_dir)
  File "/home/travis/build/facebook/watchman/build/fbcode_builder/shell_quoting.py", line 64, in format
    (k, shell_quote(v).do_not_use_raw_str) for k, v in kwargs.items()
KeyError: u'CC'
```

Reviewed By: snarkmaster

fbshipit-source-id: 614723e631a82f277739765a920731c872700c45

build/fbcode_builder/shell_builder.py

@@ -57,7 +57,7 @@ class ShellFBCodeBuilder(FBCodeBuilder):
                 ShellQuoted(
                     # Set CCACHE_DIR before the `ccache` invocations below.
                     'export CCACHE_DIR={ccache_dir} '
-                    'CC="ccache ${CC:-gcc}" CXX="ccache ${CXX:-g++}"'
+                    'CC="ccache ${{CC:-gcc}}" CXX="ccache ${{CXX:-g++}}"'
                 ).format(ccache_dir=ccache_dir)
             ]
         return steps