Rephrase sed invocation for macOS (#3724)

This PR fixes a really tiny papercut in the regression test suite; out
of the box on macOS, the version of `sed` that's installed [doesn't
understand the `+` regex
operator](https://stackoverflow.com/questions/4453760/how-to-escape-plus-sign-on-mac-os-x-bsd-sed),
which causes one test to fail by not slicing out a non-deterministic
part of some test output.

Rather than trying to install a better version of `sed` for this one
case, we just rephrase the regex in question to use `*` instead.

k-distribution/tests/regression-new/context-labels/Makefile

@@ -4,7 +4,7 @@ KOMPILE_BACKEND=haskell
 KPROVE_FLAGS+=--debugger --debug-script context-spec.k.in
 KORE_REPL_OPTS=--log-entries DebugAppliedLabeledRewriteRule
 export KORE_REPL_OPTS
-CONSIDER_PROVER_ERRORS=2>&1 | sed 's!\[[0-9]\+]!!g' # Hacking in a sed command here to remove strings of the form '[23452]'
+CONSIDER_PROVER_ERRORS=2>&1 | sed 's!\[[0-9][0-9]*]!!g' # Hacking in a sed command here to remove strings of the form '[23452]'
 
 include ../../../include/kframework/ktest.mak