chore(tools): do not use npm exec in git hooks

`npm exec` was added in npm version 7, so users of older versions of
npm will get a very weird error and are not able to commit

.husky/commit-msg

@@ -1,4 +1,4 @@
 #!/bin/sh
 . "$(dirname "$0")/_/husky.sh"
 
-npm exec commithelper -- check --file $1 --fix
+./node_modules/.bin/commithelper check --file $1 --fix

.husky/pre-commit

@@ -1,4 +1,4 @@
 #!/bin/sh
 . "$(dirname "$0")/_/husky.sh"
 
-npm exec -- lint-staged
+./node_modules/.bin/lint-staged

.husky/prepare-commit-msg

@@ -1,4 +1,4 @@
 #!/bin/sh
 . "$(dirname "$0")/_/husky.sh"
 
-exec < /dev/tty && npm exec commithelper -- prompt --file $1
+exec < /dev/tty && ./node_modules/.bin/commithelper prompt --file $1