added sccache to arch builder Docker image

arch-builder/Dockerfile

@@ -75,6 +75,7 @@ RUN mkdir -p /etc/cargo
 COPY config.toml /etc/cargo/
 RUN pacman -S --noconfirm --needed \
     rustup \
+    sccache \
     llvm \
     clang
 FROM rust AS rust-full

arch-builder/config.toml

@@ -13,3 +13,6 @@ index = "sparse+https://gitea.woggioni.net/api/packages/woggioni/cargo/"
 
 [net]
 git-fetch-with-cli = true
+
+[build]
+rustc-wrapper = /usr/bin/sccache