DT-488 Add environment variable for the AMDGPU file

When working with Snaps, a common error message is

  /usr/share/libdrm/amdgpu.ids: No such file or directory

This happens because it is not possible to include that file
directly in the root folder, but instead it must be distributed
in other snaps.

Using the "layout" statement, it is possible for an specific
snap to map that file in the root folder, but this is
cumbersome, because that forces each snap developer to manually
add that statement.

To fix this, we propose this MR, that allows to define an
environment variable, AMDGPU_ASIC_ID_TABLE_PATH, with the full
path to that file. That environment variable is used only if the
file isn't find in the original path.

This allows us to define the path of the file in the Gnome/KDE
snap, and still allow any other snap to include its own version
of libdrm, just by using the "layout" statement to put its
file in the corresponding root folder.