[seL4] [PATCH] Use usr/bin/env for all python bangpaths to enable virtualenv

This patch targets the core "seL4" repo. I idly submitted a PR on github before reading anything at all really (beyond checking to see if the issue was made more consistent in subsequent commits). Apologies if this is/was inconvenient. I saw some prior github discussion on whether "python2" or "python" is best'; IMHO, "python" is best as virtualenv allows the desired binary to be specified at env creation time. CLA forthcoming, I'm quite a long way from printers/scanners and all those sorts of things. -- @maximus_freeman 0x7D964D3361142ACF