fix(pipelines): additionalInputs not working (#17279)

The rendering of `additionalInputs` was using a bashism that is not
supported by CodeBuild by default.

Turn

```
[[ ! -d "directory" ]]
```

into

```
[ ! -d "directory" ]
```

Fixes #17224


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: rix0rrr

packages/@aws-cdk/pipelines/lib/codepipeline/_codebuild-factory.ts

@@ -333,7 +333,7 @@ function generateInputArtifactLinkCommands(artifacts: ArtifactMap, inputs: FileS
   return inputs.map(input => {
     const fragments = [];
 
-    fragments.push(`[[ ! -d "${input.directory}" ]] || { echo 'additionalInputs: "${input.directory}" must not exist yet. If you want to merge multiple artifacts, use a "cp" command.'; exit 1; }`);
+    fragments.push(`[ ! -d "${input.directory}" ] || { echo 'additionalInputs: "${input.directory}" must not exist yet. If you want to merge multiple artifacts, use a "cp" command.'; exit 1; }`);
 
     const parentDirectory = path.dirname(input.directory);
     if (!['.', '..'].includes(parentDirectory)) {

packages/@aws-cdk/pipelines/test/codepipeline/codebuild-step.test.ts

@@ -34,7 +34,7 @@ test('additionalinputs creates the right commands', () => {
         phases: {
           install: {
             commands: [
-              '[[ ! -d "some/deep/directory" ]] || { echo \'additionalInputs: "some/deep/directory" must not exist yet. If you want to merge multiple artifacts, use a "cp" command.\'; exit 1; } && mkdir -p -- "some/deep" && ln -s -- "$CODEBUILD_SRC_DIR_test2_test2_Source" "some/deep/directory"',
+              '[ ! -d "some/deep/directory" ] || { echo \'additionalInputs: "some/deep/directory" must not exist yet. If you want to merge multiple artifacts, use a "cp" command.\'; exit 1; } && mkdir -p -- "some/deep" && ln -s -- "$CODEBUILD_SRC_DIR_test2_test2_Source" "some/deep/directory"',
             ],
           },
         },

packages/@aws-cdk/pipelines/test/compliance/synths.test.ts

@@ -947,8 +947,8 @@ behavior('Multiple input sources in side-by-side directories', (suite) => {
           phases: {
             install: {
               commands: [
-                '[[ ! -d "../sibling" ]] || { echo \'additionalInputs: "../sibling" must not exist yet. If you want to merge multiple artifacts, use a "cp" command.\'; exit 1; } && ln -s -- "$CODEBUILD_SRC_DIR_foo_bar_Source" "../sibling"',
-                '[[ ! -d "sub" ]] || { echo \'additionalInputs: "sub" must not exist yet. If you want to merge multiple artifacts, use a "cp" command.\'; exit 1; } && ln -s -- "$CODEBUILD_SRC_DIR_Prebuild_Output" "sub"',
+                '[ ! -d "../sibling" ] || { echo \'additionalInputs: "../sibling" must not exist yet. If you want to merge multiple artifacts, use a "cp" command.\'; exit 1; } && ln -s -- "$CODEBUILD_SRC_DIR_foo_bar_Source" "../sibling"',
+                '[ ! -d "sub" ] || { echo \'additionalInputs: "sub" must not exist yet. If you want to merge multiple artifacts, use a "cp" command.\'; exit 1; } && ln -s -- "$CODEBUILD_SRC_DIR_Prebuild_Output" "sub"',
               ],
             },
             build: {