Add an environment variable to override cache-dir

[mbrubeck]

This will be set in servo/saltfs to prevent our buildbot builders from re-downloading the build tools for every build.

r? @larsbergstrom or @metajack or @Manishearth

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/4240

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[SimonSapin]

Can’t the builders have their own .servobuild config?

[mbrubeck]

Can’t the builders have their own .servobuild config?

Yes, that could work too. I'll see if there's a simple way to tell buildbot to copy a file from saltfs into the working dir.

[mbrubeck]

mbrubeck/saltfs@323d3c4 is an alternate fix that adds a servobuild file to the buildbot slaves, though I haven't tested it and I'm not sure if it's the best way to do that.

[larsbergstrom]

r+

[bors-servo]

saw approval from larsbergstrom
at mbrubeck@4641640

[bors-servo]

merging mbrubeck/servo/cache-dir = 4641640 into auto

[bors-servo]

mbrubeck/servo/cache-dir = 4641640 merged ok, testing candidate = 82f70c5

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/1062
success: http://build.servo.org/builders/linux2/builds/1062
success: http://build.servo.org/builders/android/builds/1096
success: http://build.servo.org/builders/mac1/builds/1278
success: http://build.servo.org/builders/mac2/builds/1278
success: http://build.servo.org/builders/gonk/builds/249

[bors-servo]

fast-forwarding master to auto = 82f70c5