Ensures make tests run under /bin/dash, like CI, and fixes a Makefile

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`.

src/test/run-make-fulldeps/coverage-reports/Makefile

@@ -1,4 +1,3 @@
-# ignore-test Broken; accidentally silently ignored on Linux CI; FIXME(#81688)
 # needs-profiler-support
 # ignore-windows-gnu
 # min-llvm-version: 11.0
@@ -128,7 +127,7 @@ endif
 			$$( \
 				for file in $(TMPDIR)/rustdoc-$@/*/rust_out; \
 				do \
-				[[ -x $$file ]] && printf "%s %s " -object $$file; \
+				[ -x "$$file" ] && printf "%s %s " -object $$file; \
 				done \
 			) \
 		2> "$(TMPDIR)"/show_coverage_stderr.$@.txt \

src/test/run-make-fulldeps/coverage/coverage_tools.mk

@@ -12,5 +12,3 @@
 # Enabling `-C link-dead-code` is not necessary when compiling with `-Z instrument-coverage`,
 # due to improvements in the coverage map generation, to add unreachable functions known to Rust.
 # Therefore, `-C link-dead-code` is no longer automatically enabled.
-
-UNAME = $(shell uname)

src/test/run-make-fulldeps/tools.mk

@@ -21,6 +21,12 @@ CGREP := "$(S)/src/etc/cat-and-grep.sh"
 # diff with common flags for multi-platform diffs against text output
 DIFF := diff -u --strip-trailing-cr
 
+# CI platforms use `/bin/dash`. When compiling in other environments, the
+# default may be different (for example, may default to `/bin/bash`), and syntax
+# and results could be different. Ensure Makefile `$(shell ...)` invocations
+# always run in `dash`.
+SHELL := /bin/dash
+
 # This is the name of the binary we will generate and run; use this
 # e.g. for `$(CC) -o $(RUN_BINFILE)`.
 RUN_BINFILE = $(TMPDIR)/$(1)