[Semaphore] Install DUB from GitHub for GDC builds

[wilzbach]

As mentioned in #7685, the gdc builds often fail with:

++ CURL_USER_AGENT='DMD-CI curl 7.35.0 (x86_64-pc-linux-gnu) libcurl/7.35.0 OpenSSL/1.0.1f zlib/1.2.8 libidn/1.28 librtmp/2.3'
++ bash install.sh gdc --activate
curl: (28) Operation too slow. Less than 1024 bytes/sec transferred the last 30 seconds
curl: (28) Operation too slow. Less than 1024 bytes/sec transferred the last 30 seconds
curl: (28) Operation too slow. Less than 1024 bytes/sec transferred the last 30 seconds
curl: (28) Operation too slow. Less than 1024 bytes/sec transferred the last 30 seconds
curl: (28) Operation too slow. Less than 1024 bytes/sec transferred the last 30 seconds
Failed to download 'http://code.dlang.org/download/LATEST'

Since dlang/installer#218, we don't download dub automatically, but use the bundled version if it's provided (DMD + LDC ship with a bundled dub binary).
I recently copied all DUB binaries from code.dlang.org to GitHub, so we can simply fetch it from there.

See also: dlang/dub#1290

CC @ibuclaw

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[wilzbach]

I think #7692 is a even better solution - no need to download dub if it isn't needed at all.