[chore] tidy up media manager, add calling func to errors, build-script improvements

[NyaaaWhatsUpDoc]

Description

• de-interfaces media.Manager
• removes now mostly unused postData callback function, instead uses a wrapped read closer when needed
• add New and Newf functions to gtserror that return an error prefixed with calling function name
• updates much code in dereferencing to use these new error types
• further simplifies the NewFromResponse() error function string building (and uses new above-mentioned errors)
• updates scripts/build.sh to support passing in Go build configuration values via env, and logs the build command

Checklist

• 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 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.

[tsmethurst]

Love this 😍