Couple of minor changes to make getting started easier from source

[oneness]

First of all, I thank you for this wonderful gem I discovered recently and has already became such an essential tool allowing me to make atomic commits a breeze.

I made following changes and hope it helps others get started with it easier :

• Made commit-patch binary take system env into account for perl binary install location. This will fix an error on machines that perl is installed in locations other than /usr/bin/perl. For example, on my M1 Mac, the location is /opt/homebrew/bin/perl.
• Added missing IPC::Run module installation note to README.md.

One side effect of this PR is that commit-patch binary will not longer has -w perl option by default. Hope you are OK with it since it can be just an alias.