Fix variable width on manual variable inputs

What does this MR do and why?

In the "run manual job" form, there are inputs to fill in variables value. When you start typing the values, a clear icon will appear next to the input. This will have the unfortunate effect of changing the width of the input field and will

Screenshots or screen recordings

Before	After
Screen_Recording_2023-01-09_at_2.40.43_PM	Screen_Recording_2023-01-09_at_3.08.58_PM

How to set up and validate locally

1. Setup a runner for your project (see: https://docs.gitlab.com/runner/register/#macos)
2. Setup a CI configuration file (.gitlab-ci.yml) by going to CI/CD => Editor
3. Note that you can have the simplest possible configuration for one single manual job like so:

job1:
  script: echo hello
  when: manual

1. Run a pipeline
2. Go to the pipeline graph
3. Click on the job action "play button" to run the manual job
4. Insert any value to the variable name
5. Press tab
6. Notice the red cross appears and does NOT make the rightmost input smaller
7. To make sure, compare with the new input that appeared below. They should be the same size! 🎉

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #377272 (closed)