Fix missing encoding of backslash in environment variable values

- Resolves microsoft#240539

src/vs/platform/terminal/common/environmentVariableCollection.ts

@@ -94,7 +94,7 @@ export class MergedEnvironmentVariableCollection implements IMergedEnvironmentVa
 	}
 
 	private _encodeColons(value: string): string {
-		return value.replaceAll(':', '\\x3a');
+		return value.replaceAll('\\', '\\\\').replaceAll(':', '\\x3a');
 	}
 
 	diff(other: IMergedEnvironmentVariableCollection, scope: EnvironmentVariableScope | undefined): IMergedEnvironmentVariableCollectionDiff | undefined {