Add PR templates and update install.sha256

[kfkonrad]

Description

This PR adds (and uses) the new pull request template for omf and updates the sha256 checksum for the install script.

It also removes a small quirk from tests/run.fish. The script will run successfully whether or not the apt plugin is installed (it installs apt to test the omf install command), but one of two strange things will happen:

1. If apt is installed prior to running the test: The warning that apt is already installed will the displayed, this may be confusing.
2. If apt is not installed prior to running the test: apt will stay on the system after the test has ended.

To fix this we now check whether apt has been installed before testing and if so remove it. This way the test will/should reinstall apt.

The test also handles the second case by removing the apt plugin if it wasn't installed before.

Fixes #687

Environment report

Oh My Fish version:   6
OS type:              Darwin
Fish version:         fish, version 3.0.2
Git version:          git version 2.21.0 Git version:          hub version 2.11.2
Git core.autocrlf:    no
Your shell is ready to swim.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing tests pass locally with my changes
• I have updated the SHA256 checksum for the install script

[sagebind]

Thanks for your help on this!