CI: create soft link after build

[kirk0830]

Reminder

• Have you linked an issue with this pull request?
• Have you added adequate unit tests and/or case tests for your pull request?
• Have you noticed possible changes of behavior below or in the linked issue?
• Have you explained the changes of codes in core modules of ESolver, HSolver, ElecState, Hamilt, Operator or Psi? (ignore if not applicable)

Linked Issue

Fix #6966

Unit Tests and/or Case Tests for my changes

• No

What's changed?

• After build, create soft link from executable abacus_2g to abacus

Any changes of core modules? (ignore if not applicable)

• No