Cache D installer download on SemaphoreCI

[wilzbach]

code.dlang.org is often down and there's no mirror for
http://code.dlang.org/download/LATEST yet

See also:

• Can we switch to hosting the DUB binaries on GitHub? dub#1290
• Use a fallback mirror for code.dlang.org installer#273
• Use fallback mirrors in the install script installer#291

CC @ibuclaw

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[ibuclaw]

Hmm, I'd consider it safe to cache the installation script and tarballs, and check for their existence before downloading. I wouldn't be so sure about extracted files though.

Couldn't say I know 100% how their cache works, but assume that any file could be removed at any point in time.

[wilzbach]

Hmm, I still see this failure very often for the gdc builds:

++ 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'

Fast-failing is activated, hence the last build fails too.

Note that since dlang/installer#218, we don't download dub automatically, but use the bundled version if it's provided (DMD + LDC do).
Well, Martin wasn't a fan of dlang/installer#273, but we could do sth. similar and simply fetch dub 1.6.0 from GitHub for gdc builds.

[wilzbach]

Downloading the install script is quite reliable with our four mirrors.
As we don't even need the dub binary, I think #7692 is the best way to move forward.