Updates `tools.mk` to explicitly require `SHELL := /bin/dash`, since CI
uses `dash` but other environments (including developer local machines)
may default to `bash`.

Replaces bash-specific shell command in one Makefile with a
dash-compatible alternative, and re-enables the affected Makefile test.

Removes apparently redundant definition of `UNAME`.

Calling an executable via full path with Windows backslashes fails in
dash.