Ensure the storage directory is calculated during runtime

[andir]

When building nimlsp in a sandbox (before installing it on a system) the temporary directory might be different. In my case I was building nimlsp with Nix which caused nimlsp to hardcode /build/nimlsp as storage directory. That directory only exists within the build sandbox and even if it would exist outside of the sandbox it wouldn't be writable by the average user.

Side note: This is the very first Nim code I've ever "written".