Ignore broken test.

(Test was silently ignored on Linux CI prior to parent commit that switched to
using `# min-llvm-version`. But the switch made the ignoring stop, exposing
other brokenness in the form of bash-dependent syntax in the `$(shell ...)`
invocations.)

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

@@ -1,3 +1,4 @@
+# ignore-test Broken; accidentally silently ignored on Linux CI; FIXME(#81688)
 # needs-profiler-support
 # ignore-windows-gnu
 # min-llvm-version: 11.0