Install newer libgit2 on RHEL 8 for git2r

[glin]

On Rocky/RHEL 8, the libgit2 version installed now is an old libgit2-0.26 from the PowerTools repo, too old for R packages like git2r. There is a newer libgit2_1.7 package in EPEL that works though, so install that instead.

dnf install -y epel-release

dnf search libgit2
# libgit2.i686 : C implementation of the Git core methods as a library with a solid API
# libgit2.x86_64 : C implementation of the Git core methods as a library with a solid API
# libgit2_1.7-devel.x86_64 : Development files for libgit2_1.7
# libgit2-glib.i686 : Git library for GLib
# libgit2-glib.x86_64 : Git library for GLib
# libgit2_1.7.x86_64 : C implementation of the Git core methods as a library with a solid API