fix(mcserver): alternative executable results in update loop and overwriting the executable.

[BertBrenner]

Description

If the executable variable is modified, fn_update_dl does not update minecraft_server.jar and which is used in fn_update_localbuild.

In addition this creates a problem if a alternative executable is used to launch minecraft by overwriting it.

Fixes N/A

Type of change

• Bug fix (a change which fixes an issue).
• New feature (a change which adds functionality).
• New Server (new server added).
• Refactor (restructures existing code).
• Comment update (typo, spelling, explanation, examples, etc).

Checklist

PR will not be merged until all steps are complete.

• This pull request links to an issue.
• This pull request uses the develop branch as its base.
• This pull request subject follows the Conventional Commits standard.
• This code follows the style guidelines of this project.
• I have performed a self-review of my code.
• I have checked that this code is commented where required.
• I have provided a detailed enough description of this PR.
• I have checked if documentation needs updating.

Documentation

If documentation does need updating either update it by creating a PR (preferred) or request a documentation update.

• User docs: https://github.com/GameServerManagers/LinuxGSM-Docs
• Dev docs: https://github.com/GameServerManagers/LinuxGSM-Dev-Docs

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