util,scons: File util/pre-commit-install error message

The backticked (`...`) pip install command in the error messages was
being parsed by the shell script as a command to run when printed. This
fixes the problem by replacing the backticks with single-quotes.

Change-Id: Ib97f6cf9f8bd0eb8bc1beae70efd5277bb51544d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/63011
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

util/pre-commit-install.sh

@@ -36,7 +36,7 @@ cd ${GEM5_ROOT}
 if ! command -v pre-commit &> /dev/null
 then
     echo "Cannot find 'pre-commit'. Please ensure all Python requirements are "
-    echo "installed. This can be done via `pip install -r requirements.txt`."
+    echo "installed. This can be done via 'pip install -r requirements.txt'."
     exit 1
 fi