Ensures `make` tests run under /bin/dash (if available), like CI, and fixes a Makefile
Note: This cherrypicks #81688 (`@pnkfelix)`
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`.
Also see: [zulip discussion thread](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/how.20to.20run.2Fbless.20src.2Ftest.2Frun-make-fulldeps.2Fcoverage.20.3F)
r? `@pnkfelix`
FYI: `@wesleywiser` `@tmandry`
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