add ccache support for custom toolchain (#180737)

Add ccache support when building the engine using a custom toolchain.

Fixes #180736

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.

engine/src/build/toolchain/custom/BUILD.gn

@@ -3,15 +3,20 @@
 # found in the LICENSE file.
 
 import("//build/toolchain/custom/custom.gni")
+import("//build/toolchain/ccache.gni")
 
 toolchain("custom") {
   toolchain_bin = "${custom_toolchain}/bin"
+  compiler_prefix = ""
+  if (use_ccache) {
+    compiler_prefix = "ccache "
+  }
 
   # We can't do string interpolation ($ in strings) on things with dots in
   # them. To allow us to use $cc below, for example, we create copies of
   # these values in our scope.
-  cc = "${toolchain_bin}/clang"
-  cxx = "${toolchain_bin}/clang++"
+  cc = "${compiler_prefix}${toolchain_bin}/clang"
+  cxx = "${compiler_prefix}${toolchain_bin}/clang++"
   ar = "${toolchain_bin}/${custom_target_triple}-ar"
   ld = "${toolchain_bin}/clang++"
   readelf = "${toolchain_bin}/${custom_target_triple}-readelf"