x/tools/gopls: enable -rpc.trace logging to a file by default

[stamblerre]

This has been discussed frequently, but I don't believe there is an issue tracking the fix. We should enable logging by default, to a file, so that users can have an easier time sharing logs with us (and don't lose them if they restart their editors). Care needs to be taken that we don't fill up temp directories with large files.

/cc @heschik @findleyr @hyangah

[hyangah]

My original motivation was not the difficulty of capturing the trace, but the difficulty of sharing the contents.
Currently, one of the common causes of many issues is that the users' workspace settings are not in the state supported by gopls well (e.g. multiple modules, etc) and the info is often at the beginning of the trace. Unfortunately, I often find users simply capture logs without the beginning of the trace, or destroy too much information during their ad-hoc anonymization.