Bump sccache used in CI to v0.2.15

This skips bumping Windows sccache because we run into compilation failures when
doing so (-m32 not supported by clang-cl?). Not clear on cause, but seems
easiest to just hold back.

This should avoid PGO-related failures encountered on Linux, and more broadly
seems like a good idea on other platforms as well (though it is likely not
necessary right this moment).

src/ci/docker/scripts/sccache.sh

@@ -1,12 +1,15 @@
 #!/bin/sh
+
+# ignore-tidy-linelength
+
 set -ex
 
 case "$(uname -m)" in
     x86_64)
-        url="https://ci-mirrors.rust-lang.org/rustc/2018-04-02-sccache-x86_64-unknown-linux-musl"
+        url="https://ci-mirrors.rust-lang.org/rustc/2021-08-24-sccache-v0.2.15-x86_64-unknown-linux-musl"
         ;;
     aarch64)
-        url="https://ci-mirrors.rust-lang.org/rustc/2019-12-17-sccache-aarch64-unknown-linux-gnu"
+        url="https://ci-mirrors.rust-lang.org/rustc/2021-08-25-sccache-v0.2.15-aarch64-unknown-linux-musl"
         ;;
     *)
         echo "unsupported architecture: $(uname -m)"

src/ci/scripts/install-sccache.sh

@@ -8,7 +8,7 @@ IFS=$'\n\t'
 source "$(cd "$(dirname "$0")" && pwd)/../shared.sh"
 
 if isMacOS; then
-    curl -fo /usr/local/bin/sccache "${MIRRORS_BASE}/2018-04-02-sccache-x86_64-apple-darwin"
+    curl -fo /usr/local/bin/sccache "${MIRRORS_BASE}/2021-08-25-sccache-v0.2.15-x86_64-apple-darwin"
     chmod +x /usr/local/bin/sccache
 elif isWindows; then
     mkdir -p sccache