[xbuild] support sccache in xbuild/xbench/xtest.

Closes: diem#6212

Author: rexhoffman

.cargo/config

@@ -6,3 +6,4 @@ xfmt = "run --package x --bin x -- fmt"
 xfix = "run --package x --bin x -- fix"
 xtest = "run --package x --bin x -- test"
 xlint = "run --package x --bin x -- lint"
+xbuild = "run --package x --bin x -- build"