Stub out the DUB download for GDC

[wilzbach]

The install script downloads dub by default if it isn't shipped by the
binary release. That's the case for old DMD releases + GDC.
So 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'

However, dub is only used for the DMD as library tests and they don't work with
gdc at the moment.

In summary: we try to fetch dub from rather unstable code.dlang.org
server and break the build due to download failures, but we don't even
need dub.

See also: #7691

[dlang-bot]

Thanks for your pull request, @wilzbach!

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