Update fails when current_exe's directory is not writable (even if current_exe is writable)

[alpire]

We're running into an issue where self update fails trying to write the tmp file in the std::env::current_exes directory. In our particular case, the std::env::current_exes directory is not writable, hence the failure. However, std::env::current_exe is writable, so it could in theory be updated.

It'd be nice if self_update could handle this case, perhaps by using /tmp if /tmp happens to be in the same filesystem as the binary being updated.

Related to #2

[jaemk]

Hi @alpire, the temp-file half of this is fixed: the download and extraction stage in the system temp dir, not in the executable's directory. The remaining case (the file is writable but its directory is not) is not implementable for a running binary. Replacement needs a rename in the target directory, and writing over the running executable's inode in place fails with ETXTBSY on Linux. What you get instead is a clear failure naming the path, optionally before anything is downloaded:

.check_install_path_writable(true)   // -> Err(Error::InstallPathNotWritable { path })