Set the default temporary directory for ZincRunner

Author: Jaden Peterson

src/main/scala/higherkindness/rules_scala/workers/zinc/compile/BUILD

@@ -4,6 +4,13 @@ load("//rules:scalafmt.bzl", "scala_format_test")
 scala_binary(
     name = "compile",
     srcs = glob(["**/*.scala"]),
+    # Per the worker protocol, workers must not write outside of the sandbox:
+    # https://bazel.build/remote/multiplex#multiplex_sandboxing
+    #
+    # Since Zinc writes temporary files to the default temporary directory, we override it to be `.` instead of `/tmp`.
+    # Normally, we'd use the sandbox directory provided in the work request, but that won't work because the default
+    # temporary directory is set globally, whereas work requests can be processed in parallel.
+    jvm_flags = ["-Djava.io.tmpdir=."],
     main_class = "higherkindness.rules_scala.workers.zinc.compile.ZincRunner",
     scala_toolchain_name = "annex_bootstrap_2_13",
     visibility = ["//visibility:public"],