chore: compat script will now also install peer dependencies (aws#12142)

In cdkv2, the package `aws-cdk-lib` has changed its dependency model to
move to use peerDependencies + devDependencies, instead of regular
dependencies.

As part of this, the `constructs` package is now a peer dependency.
jsii-diff requires this package to also be installed to load the jsii
assembly.

Change the check-api-compatibility.sh to use npm7 which installs all
declared peer dependencies as part of `npm install`.


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

scripts/check-api-compatibility.sh

@@ -68,7 +68,8 @@ if ! ${SKIP_DOWNLOAD:-false}; then
     mkdir -p $tmpdir
 
     echo "Installing from NPM..." >&2
-    (cd $tmpdir && npm install --prefix $tmpdir $existing_names)
+    # use npm7 to automatically install peer dependencies
+    (cd $tmpdir && npx npm@^7.0.0 install --prefix $tmpdir $existing_names)
 fi
 
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