gpu-compute: Use dict.get syntax for accessing buildEnv keys

gpu-compute: Use dict.get syntax for accessing buildEnv keys

37775 removed SmartDict, which is the type buildEnv used to be.
Because of that change, doing buildEnv[key] with a key not in the dict
returns KeyError instead of False. By using buildEnv(key, False), we are
able to return False when the key isn't in the dict.

Change-Id: I4aae29b95b082efb2b021f21d608f9cd1c196379
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38135
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matthew Poremba <matthew.poremba@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>