gn build: Unbreak finding a working `gn` on $PATH on Unix after r355645

From the Python subprocess docs:

   If shell is True, it is recommended to pass args as a string rather than as
   a sequence.

   [...]

   If args is a sequence, the first item specifies the command string, and any
   additional items will be treated as additional arguments to the shell itself.

Prior to this change, the `--version` would be passed to the shell, not to
a potential gn binary on $PATH, and running `gn` without any arguments makes
it exit with an exit code != 0, so the script would think that there wasn't
a working gn binary on $PATH.

Fix this by following the documentation's recommendation of using a string
now that we pass shell=True. I tested this on macOS and Windows, each with
the three cases of

- no gn on PATH (should run gn downloaded by get.py if present,
  else suggest running get.py)
- broken gn wrapper on PATH (should behave like the previous item)
- working gn on PATH (should use gn on PATH)

llvm-svn: 355694