chore: Use command -v over which for portability

[matthewfeickert]

command -v is a POSIX safe way to find an executable program. While which is very common, it still requires installation on many Linux operating systems, and so command -v can offer a more portable alternative with a very low amount of additional cognitive overhead on the maintainers.

Additionally, use a subshell, $(), over backticks for command substitution as $() is recommended for multiple reasons including readability.

c.f.:

• https://stackoverflow.com/a/4708569/8931942
• Why is $(...) preferred over ... (backticks)?
• POSIX spec for Command Substitution

I made the decision to leave which in

xrootd/docker/builds/DockerfileCentos7

Line 6 in 62d1170

RUN yum install -y gcc-c++ rpm-build which git python-srpm-macros centos-release-scl vim

and

xrootd/docker/builds/DockerfileCentos8

Line 6 in 62d1170

RUN dnf install -y gcc-c++ rpm-build which git python-srpm-macros vim dnf-plugins-core

while no longer required, as I figured that these serve as debugging environments where having which handy might be nice for alternative reasons, just as how vim is installed in them as well.

[matthewfeickert]

@simonmichal This is ready for review. I didn't open an Issue to discuss this as it seems small enough/low enough change burden to not be controversial, but I can appreciate that this could also be viewed as stylistic and so not acceptable. If so, my apologies, and feel free to close. 👍