[Runner] Fix bootstrapping

Author: giordano

src/Runner.jl

@@ -1573,7 +1573,10 @@ function runner_setup!(workspaces, mappings, workspace_root, verbose, kwargs, pl
         if !isdir(ccache_dir())
             mkpath(ccache_dir())
         end
-        push!(workspaces, ccache_dir() => envs["CCACHE_DIR"])
+        if haskey(ENV, "CCACHE_DIR")
+            # When bootstrapping, `CCACHE_DIR` is not defined.
+            push!(workspaces, ccache_dir() => envs["CCACHE_DIR"])
+        end
     end
 
     return platform, envs, shards