ci: prefetch sources

Download sources once in first step and share them as artifacts for
all other toolchain builds.  Saves time in having to download the
sources for every toolchain build.

Signed-off-by: Kumar Gala <[email protected]>

Author: galak

.buildkite/pipeline-template.yml

@@ -6,6 +6,9 @@ steps:
     - buildkite-agent artifact upload cmake.$OS.$MACHINE.tar.bz2
     - ./go.sh crosstool
     - buildkite-agent artifact upload crosstool.$OS.$MACHINE.tar.bz2
+    - export CT_ONLY_DOWNLOAD=y
+    - ./go.sh arm
+    - buildkite-agent artifact upload "build/output/sources/*"
     env:
       BUILDKITE_CLEAN_CHECKOUT: "true"
     plugins:
@@ -41,6 +44,7 @@ steps:
          16) TARGET="arc64";;
       esac
     - buildkite-agent artifact download crosstool.$OS.$MACHINE.tar.bz2 .
+    - buildkite-agent artifact download build/output/sources/* .
     - tar xvf crosstool.$OS.$MACHINE.tar.bz2 2> /dev/null
     - ./go.sh $${TARGET}
     - >