autoproj_bootstrap hangs silently when "~" is used in the PATH

We just had an isssue with autoproj_bootstrap hanging silently when started a fresh installation in line 86: `env["PATH"] = self.class.sanitize_env(ENV["PATH"] || "")`

The reason was that a "~" was used in the PATH:

> echo $PATH
[...]:~/julia-1.8.5/bin

After replacing the ~ with the full path, everything works fine.
