Packaging: add a dependency on Slurm version for RPM packages

[kcgthb]

Slurm SPANK plugins need to be recompiled for each major Slurm version. To make upgrades easier and avoid confusion, this PR proposes to add a dependency on the version of Slurm that's installed on the system, and add that major version in the generated RPM release info.

For instance, building RPMs for Pyxis 0.20 on a el9 host with Slurm 24.11 RPMs will create a nvslurm-plugin-pyxis-0.20.0-1.slurm2411.el9.x86_64.rpm package, with a dependency on libslurm.so.42()(64bit).

To do this, we get the version of the installed slurm-devel RPM package in the SPEC file, via a rpm -q command. Doing this uses the "%{VERSION}" RPM tag, which conflicts with VERSION variable that is used in the Makefile and passed to rpmbuild. So to avoid unwanted expansion of that %{VERSION} variable, it's been renamed to PYXIS_VER in the Makefile.