GlideinWMS

The current order, after [#25217], is:

1. Optional: Look first in the override path (GLIDEIN_SINGULARITY_BINARY_OVERRIDE)
2. Optional: Look in the path suggested via SINGULARITY_BIN (keyword OSG -> OSG location)
3. Look in the default OSG location
4. Look in $PATH
5. Invoke module singularitypro
6. Invoke module singularity

If SINGULARITY_BIN contains OSG, it looks in the OSG location, same as the following step.

The idea would be to have OSG be the default value of SINGULARITY_BIN and change the order of the tests:

1. Optional: Look first in the override path (GLIDEIN_SINGULARITY_BINARY_OVERRIDE)
2. Optional: Look in the path suggested via SINGULARITY_BIN (keyword OSG -> OSG location, this would be the default)
3. Look in $PATH
4. Invoke module singularitypro
5. Invoke module singularity
6. Look in the default OSG location

This way there would be no practical change by default, but a change in the SINGULARITY_BIN would allow the Factory/Frontend to cause the PATH and module to be looked at first, before the OSG location. Maybe a keyword like SKIP or PATH could be used to avoid the warning/debug messages.

The ability to specify a path could remain undocumented or be removed, leaving SINGULARITY_BIN only as a toggle.