[feature] Add support for custom go binary name for build.sh

[hellium6]

Description

I apologize I couldn't open an issue. So I'm submitting this PR. This is a small change, so I hope it's ok.

On FreeBSD, for example, go binary installed from repo has different name (not go as the script currently assumes):

$ uname -rs
FreeBSD 14.1-RELEASE-p5
$ which go
$ which go121
/usr/local/bin/go121

So running build.sh shows env: go: No such file or directory.

This PR allows passing GO_BIN to solve it:

GO_BIN="go121" GO_BUILDTAGS="moderncsqlite3" scripts/build.sh

closes #(issue)
closes #(another issue)

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.