Set/unset USE_LXC, USE_VBOX, USE_DOCKER explicitly

This prevents the setting of more than one environment variable for the
gitian-builder (e.g., USE_LXC being set shadows USE_DOCKER; for details
see gitian-builder/libexec/make-clean-vm).

contrib/gitian-build.py

@@ -149,7 +149,7 @@ def main():
     parser.add_argument('-m', '--memory', dest='memory', default='2000', help='Memory to allocate in MiB. Default %(default)s')
     parser.add_argument('-k', '--kvm', action='store_true', dest='kvm', help='Use KVM instead of LXC')
     parser.add_argument('-d', '--docker', action='store_true', dest='docker', help='Use Docker instead of LXC')
-    parser.add_argument('-S', '--setup', action='store_true', dest='setup', help='Set up the Gitian building environment. Uses LXC. If you want to use KVM, use the --kvm option. Only works on Debian-based systems (Ubuntu, Debian)')
+    parser.add_argument('-S', '--setup', action='store_true', dest='setup', help='Set up the Gitian building environment. Only works on Debian-based systems (Ubuntu, Debian)')
     parser.add_argument('-D', '--detach-sign', action='store_true', dest='detach_sign', help='Create the assert file for detached signing. Will not commit anything.')
     parser.add_argument('-n', '--no-commit', action='store_false', dest='commit_files', help='Do not commit anything to git')
     parser.add_argument('signer', help='GPG signer to sign each build assert file')
@@ -173,14 +173,18 @@ def main():
 
     args.sign_prog = 'true' if args.detach_sign else 'gpg --detach-sign'
 
-    # Set environment variable USE_LXC or USE_DOCKER, let gitian-builder know that we use lxc or docker
+    # Ensure no more than one environment variable for gitian-builder (USE_LXC, USE_VBOX, USE_DOCKER) is set as they
+    # can interfere (e.g., USE_LXC being set shadows USE_DOCKER; for details see gitian-builder/libexec/make-clean-vm).
+    os.environ['USE_LXC'] = ''
+    os.environ['USE_VBOX'] = ''
+    os.environ['USE_DOCKER'] = ''
     if args.docker:
         os.environ['USE_DOCKER'] = '1'
     elif not args.kvm:
         os.environ['USE_LXC'] = '1'
-        if not 'GITIAN_HOST_IP' in os.environ.keys():
+        if 'GITIAN_HOST_IP' not in os.environ.keys():
             os.environ['GITIAN_HOST_IP'] = '10.0.3.1'
-        if not 'LXC_GUEST_IP' in os.environ.keys():
+        if 'LXC_GUEST_IP' not in os.environ.keys():
             os.environ['LXC_GUEST_IP'] = '10.0.3.5'
 
     # Disable for MacOS if no SDK found