build.d: Make download fail reliably

[MoonlightSentinel]

download would sometimes return true on failure (caused by Issue 18318 which requires additional work in dub). This PR implements a temporary workaround and should be reverted when the issue is resolved.

See https://issues.dlang.org/show_bug.cgi?id=18318

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	18318	major	std.net.curl.download silently ignores non-2xx http statuses

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#10590"

[thewilsonator]

Auto-merge toggled on