Add shellcheck to pre-commit and fix warnings

[gforsyth]

shellcheck is a fast, static analysis tool for shell scripts. It's good at
flagging up unused variables, unintentional glob expansions, and other potential
execution and security headaches that arise from the wonders of bash (and
other shlangs).

This PR adds a pre-commit hook to run shellcheck on all of the sh-lang
files in the ci/ directory, and the changes requested by shellcheck to make
the existing files pass the check.

xref: rapidsai/build-planning#135