Fix the prepare source shell script and add additional steps to the build workflow

[sachinh-amazon]

Description of changes

1. Fix bugs in the prepare source bash script related to paths.
2. Simplify the script - initially the script accepted an additional command line argument for the folder path which will contain the patched source. Now, the script will always create the patched source in the code-editor-src folder.
3. Extend the build workflow Github action to install quilt and run the bash script for preparing the patched source.

Testing

1. Ran the build Github action from and verified that it works as expected. Link: https://github.com/aws/code-editor/actions/runs/16770307655
2. Tested locally and verified that the patched source is getting created in the correct directory.

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