[Issue 21226] Bug in installer.sh#L552-L556 and missing signature files

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21226

--- Comment #1 from Dlang Bot <dlang-bot at dlang.rocks> ---
@wilzbach created dlang/installer pull request #470 "Issue 21226 - Support
installing older DMD versions" mentioning this issue:

- Issue 21226 - Support installing older DMD versions

https://github.com/dlang/installer/pull/470

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