ci: reduce root-reserve-mb size for maximize-build-space

[DmitriyLewen]

Description

Size of GitHub action ubuntu image has been reduce:
before:

Filesystem Size Used Avail Use% Mounted on 
/dev/root 84G 53G 31G 64% /

now:

Filesystem Size Used Avail Use% Mounted on 
/dev/root 73G 53G 21G 73% /

So we can't get 35840MB - https://github.com/aquasecurity/trivy/actions/runs/7779351592/job/21210359409
So we need to reduce root-reserve-mb value.

Also i played with root-reserve-mb. Size equal 32GB doesn't return error.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[knqyf263]

Great!