install: handle GitHub API JSON without newlines

[bendrucker]

Handles GitHub API responses that do not include newlines. Previously, if the releases/latest response lacked newlines within its JSON, get_latest_release would return mentions_count since that is the last double-quoted string on the line grep matched (the whole object).

Now, sed will only capture the string value after "tag_name". It looks past a colon and optional surrounding whitespace and captures the next double quoted group. This is compatible with both compact and multiline JSON.

We still need grep to handle multiline JSON, since we need to exclude all lines other than the one we want. Otherwise the output is the original object with a single malformed line containing the latest version. GNU sed can handle multiline input with -z so keeping grep for that preserves portability.

Closes #2175

[wata727]

👍