fix regex for extracting glibc version from output of 'ldd --version' in Gentoo Linux

[boegel]

Without this change, we get UNKNOWN in a Gentoo Linux environment:

$ eb --show-system-info | grep glibc
  -> glibc version: UNKNOWN

That causes trouble with Qt5 for example, because get_glibc_version is used in the sanity check, which leads to:

  File "/cvmfs/pilot.eessi-hpc.org/versions/2023.06/software/linux/x86_64/generic/software/EasyBuild/4.7.2/lib/python3.11/site-packages/easybuild/easyblocks/q/qt.py", line 203, in sanity_check_step
    if LooseVersion(glibc_version) <= LooseVersion("2.16"):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/cvmfs/pilot.eessi-hpc.org/versions/2023.06/compat/linux/x86_64/usr/lib/python3.11/site-packages/setuptools/_distutils/version.py", line 78, in __le__
    c = self._cmp(other)
        ^^^^^^^^^^^^^^^^
  File "/cvmfs/pilot.eessi-hpc.org/versions/2023.06/compat/linux/x86_64/usr/lib/python3.11/site-packages/setuptools/_distutils/version.py", line 351, in _cmp
    if self.version < other.version:
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: '<' not supported between instances of 'str' and 'int'

(cfr. EESSI/software-layer#284)

With this change, the glibc version is detected correctly also in Gentoo:

[EESSI pilot 2023.06] $ ldd --version
ldd (Gentoo 2.37-r3 (patchset 5)) 2.37
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Roland McGrath and Ulrich Drepper.

[EESSI pilot 2023.06] $ eb --show-system-info | grep glibc
  -> glibc version: 2.37