Misc script fixes

• Fix shebangs in scripts

  Only /bin/sh and /usr/bin/env exist almost everywhere and work consistently.

  On some systems, the programs installed at /bin or /usr/bin might not be suitable versions. In fact, they may not even exist.

• trackertestutils: Use better program name in command-line help

  The tracker-sandbox script was reporting its name as __main__. This changes it to "tracker-sandbox".

  Additionally, it respects the argv[0] parameter given to execv(), so the tracker-miners/run-uninstalled script can set its program name too. (See tracker-miners!492 (merged))

• tracker-sandbox: Fix PYTHONPATH

  It was failing to find the gi module for me.