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

component/glib/buildout.cfg

@@ -16,7 +16,7 @@ shared = true
 url = https://ftp.gnome.org/pub/gnome/sources/glib/2.56/glib-2.56.4.tar.xz
 md5sum = 17c3dca43d99a4882384f1a7b530b80b
 configure-options =
-  --with-python=${buildout:executable}
+  --with-python=python
   --disable-libmount
   --disable-static
   --disable-selinux