Add RUSTFLAGS = aws_sdk_unstable as environment variable when running Cargo from kotlin (#2614)

## Motivation and Context
This PR set `aws_sdk_unstable` to RUSTFALGS when running cargo from
kotlin.

It is required to enable test gated features introduced on RFC30.

No breaking changers are introduced.

## Description
add `RUSTFLAGS = aws_sdk_unstable` when running cargo from kotlin.


## Parent PR
This PR is listed as one of prerequisite PRs on this PR.

- #2615

## Testing
NA.

----
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choice.*

Author: thomas-k-cameron

buildSrc/src/main/kotlin/CodegenTestCommon.kt

@@ -244,25 +244,29 @@ fun Project.registerCargoCommandsTasks(
     this.tasks.register<Exec>(Cargo.CHECK.toString) {
         dependsOn(dependentTasks)
         workingDir(outputDir)
+        environment("RUSTFLAGS", "--cfg aws_sdk_unstable")
         commandLine("cargo", "check", "--lib", "--tests", "--benches", "--all-features")
     }
 
     this.tasks.register<Exec>(Cargo.TEST.toString) {
         dependsOn(dependentTasks)
         workingDir(outputDir)
+        environment("RUSTFLAGS", "--cfg aws_sdk_unstable")
         commandLine("cargo", "test", "--all-features")
     }
 
     this.tasks.register<Exec>(Cargo.DOCS.toString) {
         dependsOn(dependentTasks)
         workingDir(outputDir)
         environment("RUSTDOCFLAGS", defaultRustDocFlags)
+        environment("RUSTFLAGS", "--cfg aws_sdk_unstable")
         commandLine("cargo", "doc", "--no-deps", "--document-private-items")
     }
 
     this.tasks.register<Exec>(Cargo.CLIPPY.toString) {
         dependsOn(dependentTasks)
         workingDir(outputDir)
+        environment("RUSTFLAGS", "--cfg aws_sdk_unstable")
         commandLine("cargo", "clippy")
     }
 }

buildSrc/src/main/kotlin/RustBuildTool.kt

@@ -28,6 +28,7 @@ private fun runCli(
                 }
             }
             .copyTo(action)
+        action.environment("RUSTFLAGS", "--cfg aws_sdk_unstable")
         action.execute()
     }
 }