Set edition for process_wrapper and cargo_build_script_runner (#1254)

These otherwise get built with the downstream repo's default edition,
which is not necessarily correct.
diff --git a/cargo/cargo_build_script_runner/BUILD.bazel b/cargo/cargo_build_script_runner/BUILD.bazel
index 9a7523d..bf72b2a 100644
--- a/cargo/cargo_build_script_runner/BUILD.bazel
+++ b/cargo/cargo_build_script_runner/BUILD.bazel
@@ -3,16 +3,19 @@
 rust_library(
     name = "cargo_build_script_output_parser",
     srcs = ["lib.rs"],
+    edition = "2018",
 )
 
 rust_test(
     name = "test",
     crate = ":cargo_build_script_output_parser",
+    edition = "2018",
 )
 
 rust_binary(
     name = "cargo_build_script_runner",
     srcs = ["bin.rs"],
+    edition = "2018",
     visibility = ["//visibility:public"],
     deps = [":cargo_build_script_output_parser"],
 )
@@ -20,6 +23,7 @@
 rust_test(
     name = "bin_test",
     crate = ":cargo_build_script_runner",
+    edition = "2018",
     deps = [":cargo_build_script_runner"],
 )
 
diff --git a/util/process_wrapper/BUILD.bazel b/util/process_wrapper/BUILD.bazel
index ebceb4a..3a2b288 100644
--- a/util/process_wrapper/BUILD.bazel
+++ b/util/process_wrapper/BUILD.bazel
@@ -6,12 +6,14 @@
 rust_binary_without_process_wrapper(
     name = "process_wrapper",
     srcs = glob(["*.rs"]),
+    edition = "2018",
     visibility = ["//visibility:public"],
 )
 
 rust_test(
     name = "process_wrapper_test",
     crate = ":process_wrapper",
+    edition = "2018",
 )
 
 filegroup(