Makefile.toml: fix check-shell regex

Fixes a warning from newer versions of grep:
  grep: warning: stray \ before !

Signed-off-by: Ben Cressey <[email protected]>

Author: bcressey

twoliter/embedded/Makefile.toml

@@ -517,7 +517,7 @@ if ! docker run --rm \
   bash -c \
     'flagged_scripts=0 && \
     cd /tmp/tools && \
-    for shell_script in $(grep -l --directories=skip --regexp="^#\!.*bash$" * ); do
+    for shell_script in $(grep -l --directories=skip --regexp='^#!.*bash$' * ); do
       if ! shellcheck \
         --external-sources \
         --source-path=SCRIPTDIR \