Download logic is broken with manifest overrides.

[davecgh]

Whenever pointing the installer to a different manifest file, the logic tries to download the associated binary from the same path the manifest was obtained from and there is no way to override it.

This does not work well when you need to use a modified manifest.

For example, try the following:

• Grab the Bitcoin 0.20.1 manifest from here
• Modify it to remove the url from the top of the file so it matches the data that is committed to via the overall v1.6.2 manifest.
• Run the installer pointing to the overridden bitcoin manifest:
  dcrinstall-windows-amd64-v1.6.2.exe -bitcoinmanifest=file://bitcoin-core-0.20.1-SHA256SUMS.asc.txt -dcrdex
• Observe that it attempts to download Download file: file://bitcoin-0.20.1-win64.zip which doesn't exist unless you also already manually downloaded it and put it there.