Respect the fileextension with roc build --lib --output file.extension

[ostcar]

When you call roc build --lib --output file.EXTENSION, the fileextension is changed to .so. This is as least true for linux.

The reason is this line: https://github.com/roc-lang/roc/blob/62cc19c64b65d2f652d36a06016921f2f8faf183/crates/compiler/build/src/link.rs#L979

This can not be changed without changes on other parts of roc. For example this code expects a filename with a .so extension on target linux and other specific fileextensions for other targets.

• See this discussion on zulip
• This old PR with other places, where a change of the filename did mater.
• This old PR that tried to fix this.