fix some moved code from revert of aosp/2518976

aosp/2768553 reverted aosp/2518976, but there was some code that was
moved in between these two commits. This commit removes a code block
for some envvars that were moved in aosp/2638675, and adds back the code
for the ANDROID_RUST_DARWIN envvar.

Change-Id: I9ac09c608fe64340535ac493ab7f07ecb9b9c335
diff --git a/rust/builder.go b/rust/builder.go
index fe2d03a..72e5be1 100644
--- a/rust/builder.go
+++ b/rust/builder.go
@@ -238,6 +238,10 @@
 		}
 	}
 
+	if ctx.Darwin() {
+		envVars = append(envVars, "ANDROID_RUST_DARWIN=true")
+	}
+
 	return envVars
 }
 
@@ -346,19 +350,6 @@
 		implicits = append(implicits, outputs.Paths()...)
 	}
 
-	envVars = append(envVars, "ANDROID_RUST_VERSION="+config.GetRustVersion(ctx))
-
-	if ctx.RustModule().compiler.CargoEnvCompat() {
-		if _, ok := ctx.RustModule().compiler.(*binaryDecorator); ok {
-			envVars = append(envVars, "CARGO_BIN_NAME="+strings.TrimSuffix(outputFile.Base(), outputFile.Ext()))
-		}
-		envVars = append(envVars, "CARGO_CRATE_NAME="+ctx.RustModule().CrateName())
-		pkgVersion := ctx.RustModule().compiler.CargoPkgVersion()
-		if pkgVersion != "" {
-			envVars = append(envVars, "CARGO_PKG_VERSION="+pkgVersion)
-		}
-	}
-
 	if flags.Clippy {
 		clippyFile := android.PathForModuleOut(ctx, outputFile.Base()+".clippy")
 		ctx.Build(pctx, android.BuildParams{