fixup optional crossbeam feature selection in debouncer-mini

[0xpr03]

Actually pass through the optional crossbeam dep from debouncer-mini to notify.

Also verify this in CI. (Requires a manual -p call to work.)

[0xpr03]

The bad part: if we want to name the feature crossbeam-channel, as used in notify, we'll have to raise the MSRV to 1.60. So this is my workaround.

[0xpr03]

CC @JohnTitor for pro/contra