bug_report.md: use LC_ALL instead of LANG

This overrides all `LC_*` variables (and LANG) rather than just LANG.

From Section 8.2, Internationalization Variables of POSIX.1-2017[1]:

> LANG
>     This variable shall determine the locale category for native
>     language, local customs, and coded character set in the absence of
>     the LC_ALL and other LC_* (LC_COLLATE, LC_CTYPE, LC_MESSAGES,
>     LC_MONETARY, LC_NUMERIC, LC_TIME) environment variables.  This can
>     be used by applications to determine the language to use for error
>     messages and instructions, collating sequences, date formats, and
>     so on.
>
> LC_ALL
>     This variable shall determine the values for all locale
>     categories.  The value of the LC_ALL environment variable has
>     precedence over any of the other environment variables starting
>     with LC_ (LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY,
>     LC_NUMERIC, LC_TIME) and the LANG environment variable.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html

.github/ISSUE_TEMPLATE/bug_report.md

@@ -15,7 +15,7 @@ _Describe the bug_
 
 _Steps to reproduce the behavior_
 
-1. Run in bash `LANG=C firejail PROGRAM` (`LANG=C` to get English messages that can be understood by everybody)
+1. Run in bash `LC_ALL=C firejail PROGRAM` (`LC_ALL=C` to get a consistent output in English that can be understood by everybody)
 2. Click on '....'
 3. Scroll down to '....'
 4. See error `ERROR`