servo: Merge #19883 - Re-enable sccache for Linux builds (from aneeshusa:reenable-sccache-partially); r=jdm

As far as I know, sccache is working properly
on the non-cross-compiling Linux builders.
For safety, only enable it for the builders that run on PRs,
to avoid breaking our nightly generation and scheduled test runs.

This will also allow testing new versions of sccache more easily.
This implements my suggestion from servo/servo#19858 (comment), and should also let us
handle testing a new sccache: rust-lang/rust#42867 (comment) (our current version of sccache [seems to be 2018-01-09](https://github.com/servo/saltfs/blob/f50214b8fa012e03616ecae1ef2913e6fe9044da/servo-build-dependencies/ci-map.jinja#L5)).

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Source-Repo: https://github.com/servo/servo
Source-Revision: 78ffce1cbe5fcce4d057b69c3cbf0cd2bc2b449c

UltraBlame original commit: d897da50fc624ee27af7c16c02e368b92f4a6f5e

Author: marco-c

servo/etc/ci/buildbot_steps.yml

@@ -1672,6 +1672,16 @@ linux
 -
 dev
 :
+env
+:
+CCACHE
+:
+sccache
+RUSTC_WRAPPER
+:
+sccache
+commands
+:
 -
 .
 /
@@ -1840,6 +1850,16 @@ rel
 -
 wpt
 :
+env
+:
+CCACHE
+:
+sccache
+RUSTC_WRAPPER
+:
+sccache
+commands
+:
 -
 .
 /
@@ -2043,6 +2063,16 @@ rel
 -
 css
 :
+env
+:
+CCACHE
+:
+sccache
+RUSTC_WRAPPER
+:
+sccache
+commands
+:
 -
 .
 /