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 <d2406197fb1bedd1e23c70c50c510ff0e9ab0f5b@gmail.com>
Reviewed-by: Matthew Poremba <f1eb50f3368ce197c52b19f7fdb5aa7a278f15fd@amd.com>
Maintainer: Matt Sinclair <d2406197fb1bedd1e23c70c50c510ff0e9ab0f5b@gmail.com>
Maintainer: Matthew Poremba <f1eb50f3368ce197c52b19f7fdb5aa7a278f15fd@amd.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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