fix: update GitHub Actions workflows

[michaelawyu]

What type of PR is this?

/kind bug
/kind test
/kind failing-test

What this PR does / why we need it:

This PR updates the GitHub Actions workflows for the fleet-networking repository, as a number of workflows no longer function after the repository went public. Specifically:

• Trivy vulnerability scanner are set to run only when a) a PR is merged to the main branch, b) a new release tag is made -> for security reasons, on public repositories, GitHub Actions will not grant write permissions to workflow runs upon new pull requests
• E2E tests are set to run only when a) a PR is merged to the main branch, b) a new release tag is made, or c) requested explicitly -> for security reasons, on public repositories, GitHub Actions will not pass in secrets to workflow runs upon new pull requests
• Proxy settings for accessing private Go modules are removed
• Minor fixes/adjustments

Which issue(s) this PR fixes:

N/A

Requirements:

• [N/A] run make reviewable for basic local test
• Read and followed fleet-networking's Code of conduct.
• uses conventional commit messages
• [N/A] includes documentation
• [N/A] adds unit tests

How has this code been tested

N/A

Special notes for your reviewer

[codecov]

Codecov Report

Merging #146 (4999b47) into main (96d65ab) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
+ Coverage   78.28%   78.31%   +0.03%     
==========================================
  Files          20       20              
  Lines        2712     2712              
==========================================
+ Hits         2123     2124       +1     
+ Misses        468      467       -1     
  Partials      121      121              

Impacted Files	Coverage Δ
pkg/controllers/multiclusterservice/controller.go	84.17% <0.00%> (-1.02%)	⬇️
...ontrollers/hub/internalserviceimport/controller.go	77.38% <0.00%> (+1.58%)	⬆️

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