glib: put the expected python in $PATH

On testnodes, python can be too long for linux shebang (#!) line.

We had for example:
    
    /srv/slapgrid/slappart15/srv/testnode/cqc/inst/test0-0/tmp/shared/python2.7/338ccc0e387d64185920266f8a97c87b/bin/python2.7
  

that is 123 characters long.

With this, the generated glib-genmarshall script will have:
    
    #!/usr/bin/env  /srv/slapgrid/slappart15/srv/testnode/cqc/inst/test0-0/tmp/shared/python2.7/338ccc0e387d64185920266f8a97c87b/bin/python2.7

which exceeds the linux kernel limit.

Instead, we generate scripts that will use
    
    #!/usr/bin/env python

and it will be the caller responsibility to set $PATH to use the expected python.


This fixes the issues we had on SlapOS.SoftwareReleases.IntegrationTest tests since we updated glib to 2.56.4 in a7a86db9

/reviewed-on nexedi/slapos!670