Bug 1690937 - Build sccache with a sysroot. r=firefox-build-system-reviewers,sheehan,mhentges

We can't use the system compiler (gcc) because of
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=786883
(which despite being closed, still is a problem)
We could use the linux64-gcc toolchain, but since we're using clang
on other platforms, we might as well do the same.

Differential Revision: https://phabricator.services.mozilla.com/D104129

taskcluster/ci/toolchain/sccache.yml

@@ -21,13 +21,16 @@ linux64-sccache:
         symbol: TL(sccache)
     worker-type: b-linux
     worker:
+        docker-image: {in-tree: deb10-toolchain-build}
         max-run-time: 1800
     run:
         toolchain-artifact: public/build/sccache.tar.xz
     fetches:
         toolchain:
             - linux64-rust-1.47
+            - linux64-clang-11
             - linux64-binutils
+            - linux64-sysroot
 
 macosx64-sccache:
     treeherder:

taskcluster/scripts/misc/build-sccache.sh

@@ -33,14 +33,16 @@ COMMON_FEATURES="native-zlib"
 
 case "$(uname -s)" in
 Linux)
+    export CC="$MOZ_FETCHES_DIR/clang/bin/clang"
     if [ "$TARGET" == "x86_64-apple-darwin" ]; then
         export PATH="$MOZ_FETCHES_DIR/llvm-dsymutil/bin:$PATH"
         export PATH="$MOZ_FETCHES_DIR/cctools/bin:$PATH"
         export RUSTFLAGS="-C linker=$GECKO_PATH/taskcluster/scripts/misc/osx-cross-linker"
-        export CC="$MOZ_FETCHES_DIR/clang/bin/clang"
         export TARGET_CC="$MOZ_FETCHES_DIR/clang/bin/clang -isysroot $MOZ_FETCHES_DIR/MacOSX10.12.sdk"
         cargo build --features "all $COMMON_FEATURES" --verbose --release --target $TARGET
     else
+        export CFLAGS_x86_64_unknown_linux_gnu="--sysroot=$MOZ_FETCHES_DIR/sysroot"
+        export RUSTFLAGS="-C linker=$CC -C link-arg=--sysroot=$MOZ_FETCHES_DIR/sysroot"
         cargo build --features "all dist-server openssl/vendored $COMMON_FEATURES" --verbose --release
     fi