scripts: Use portable paste invocation in language check (#5)

### Motivation

The script that checks for unacceptable language used an invocation of
`paste` that worked on Linux (i.e. in the CI), but not with the variant
of `paste` preinstalled on Darwin platforms.

This meant it failed like this:

```console
❯ bash scripts/check-for-unacceptable-language.sh
** Checking for unacceptable language...
usage: paste [-s] [-d delimiters] file ...
```

Worse, the `|| true` was in the wrong place so it failed silently.

### Modifications
- Use portable invocation of `paste` for Linux and Darwin.
- Move the `|| true` so that, when it fails, it fails hard.

### Result

Can run the script on Darwin.

### Testing

```
❯ bash scripts/check-for-unacceptable-language.sh
** Checking for unacceptable language...
** ✅ Found no unacceptable language.

❯ echo kill  >> README.md

❯ bash scripts/check-for-unacceptable-language.sh
** Checking for unacceptable language...
** ERROR: ❌ Found unacceptable language in files: README.md.
```

Signed-off-by: Si Beaumont <beaumont@apple.com>

Author: simonjbeaumont

scripts/check-for-unacceptable-language.sh

@@ -28,8 +28,7 @@ PATHS_WITH_UNACCEPTABLE_LANGUAGE=$(git -C "${REPO_ROOT}" grep \
   -f "${UNACCEPTABLE_LANGUAGE_PATTERNS_PATH}" \
   -- \
   ":(exclude)${UNACCEPTABLE_LANGUAGE_PATTERNS_PATH}" \
-  | paste -s -d " "
-) ||:
+) || true | /usr/bin/paste -s -d " " -
 
 if [ -n "${PATHS_WITH_UNACCEPTABLE_LANGUAGE}" ]; then
   fatal "❌ Found unacceptable language in files: ${PATHS_WITH_UNACCEPTABLE_LANGUAGE}."