Avoid ./mach build to clobber the build by default.

[emilio]

Fixes #20489.

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This change is 

[highfive]

Heads up! This PR modifies the following files:

• @wafflespeanut: python/servo/command_base.py
• @edunham: .travis.yml
• @aneeshusa: etc/ci/buildbot_steps.yml

[emilio]

r? @jdm

@bors-servo try

[bors-servo]

⌛ Trying commit d2cdfff with merge c7bcd16...

[emilio]

Not sure if the WITH_GOLD bit will make the ARM build fail... I guess we'll see.

[jdm]

Not sure if the tradeoff is worth disabling the gold linker by default :/

[bors-servo]

☀️ Test successful - android, arm32, arm64, linux-dev, linux-rel-css, linux-rel-wpt, mac-dev-unit, mac-rel-css1, mac-rel-css2, mac-rel-wpt1, mac-rel-wpt2, mac-rel-wpt3, mac-rel-wpt4, windows-msvc-dev
State: approved= try=True

[emilio]

Not sure if the tradeoff is worth disabling the gold linker by default :/

It definitely is for me, but if you think otherwise I can try to find another solution... Not clear to me how'd that look.

[jdm]

It might be worth taking this to the mailing list. I'm on leave this month, so I'm unassigning myself.

[nox]

@emilio What's the status on this?

[emilio]

I mailed the list. Not sure if we should proceed here or not, I guess @SimonSapin tried to make the gold bits use the cargo config and hit a cargo bug.

[SimonSapin]

My hope was to move the "unused extern crate" warning to .cargo/config (and while I’m at it the "enable NEON on ARM" bit too, since .cargo/config can have cfg(…) sections), and keep the linker one set by mach in an env variable because it’s done dynamically based on the presence of ld.gold in $PATH. I was hoping that both sets of flags would be passed to rustc, but instead the env variable overrides the config: rust-lang/cargo#5376

[jdm]

Given the downsides, I'm going to close this.