Remove potential bashism, replace with something more POSIX-y

Signed-off-by: Marek Kubica <[email protected]>

Author: Leonidas-from-XIV

test/blackbox-tests/test-cases/pkg/ocamlformat/ocamlformat-custom-build-dir.t

@@ -18,7 +18,7 @@ the build directory in `$dev_tool_lock_dir` with our custom build directory.
   $ default_build_dir="_build"
   $ custom_build_dir="_other_build"
   $ default_dev_tool_lock_dir="${dev_tool_lock_dir}"
-  $ dev_tool_lock_dir="${dev_tool_lock_dir/#$default_build_dir/$custom_build_dir}"
+  $ dev_tool_lock_dir=$(echo "${dev_tool_lock_dir}" | sed "s/^$default_build_dir/$custom_build_dir/")
 
 Create a configuration with this custom build directory