fix(ngcc): only back up the original prepublishOnly script and not the overwritten one (#32427)

In order to prevent `ngcc`'d packages (e.g. libraries) from getting
accidentally published, `ngcc` overwrites the `prepublishOnly` npm
script to log a warning and exit with an error. In case we want to
restore the original script (e.g. "undo" `ngcc` processing), we keep a
backup of the original `prepublishOnly` script.

Previously, running `ngcc` a second time (e.g. for a different format)
would create a backup of the overwritten `prepublishOnly` script (if
there was originally no `prepublishOnly` script). As a result, if we
ever tried to "undo" `ngcc` processing and restore the original
`prepublishOnly` script, the error-throwing script would be restored
instead.

This commit fixes it by ensuring that we only back up a `prepublishOnly`
script, iff it is not the one we created ourselves (i.e. the
error-throwing one).

PR Close #32427

packages/compiler-cli/ngcc/test/packages/build_marker_spec.ts

@@ -147,6 +147,26 @@ runInEachFileSystem(() => {
         expect(pkg.scripts.prepublishOnly).toContain('This is not allowed');
         expect(pkg.scripts.prepublishOnly__ivy_ngcc_bak).toBe(prepublishOnly);
       });
+
+      it(`should not keep backup of overwritten 'prepublishOnly' script`, () => {
+        const COMMON_PACKAGE_PATH = _('/node_modules/@angular/common/package.json');
+        const fs = getFileSystem();
+        let pkg = JSON.parse(fs.readFile(COMMON_PACKAGE_PATH));
+
+        markAsProcessed(fs, pkg, COMMON_PACKAGE_PATH, ['fesm2015']);
+
+        pkg = JSON.parse(fs.readFile(COMMON_PACKAGE_PATH));
+        expect(pkg.scripts.prepublishOnly).toContain('This is not allowed');
+        expect(pkg.scripts.prepublishOnly__ivy_ngcc_bak).toBeUndefined();
+
+        // Running again, now that there is `prepublishOnly` script (created by `ngcc`), it should
+        // still not back it up as `prepublishOnly__ivy_ngcc_bak`.
+        markAsProcessed(fs, pkg, COMMON_PACKAGE_PATH, ['fesm2015']);
+
+        pkg = JSON.parse(fs.readFile(COMMON_PACKAGE_PATH));
+        expect(pkg.scripts.prepublishOnly).toContain('This is not allowed');
+        expect(pkg.scripts.prepublishOnly__ivy_ngcc_bak).toBeUndefined();
+      });
     });
 
     describe('hasBeenProcessed', () => {