osg-ca-certs: avoid StandardVersion -> str warning

[wdconinc]

This PR fixes osg-ca-certs to avoid the following warning:

#29 230.9 > ==> Warning: /opt/spack-packages/repos/spack_repo/builtin/packages/osg_ca_certs/package.py:59: when setting environment variable OSG_CERTS_VERSION=1.141: value is of type `StandardVersion`, but `str` was expected. This is deprecated and will be an error in Spack v1.0
#29 230.9 > ==> Warning: /opt/spack-packages/repos/spack_repo/builtin/packages/osg_ca_certs/package.py:61: when setting environment variable IGTF_CERTS_VERSION=1.141: value is of type `StandardVersion`, but `str` was expected. This is deprecated and will be an error in Spack v1.0

[Copilot]

Pull request overview

This PR updates the osg-ca-certs Spack package to avoid Spack warnings (and future Spack v1.0 errors) caused by passing StandardVersion values to EnvironmentModifications.set(), which expects str.

Changes:

• Convert self.version[:2] to str(...) when setting OSG_CERTS_VERSION.
• Convert self.version[3:] to str(...) when setting IGTF_CERTS_VERSION.

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