[vcpkg scripts] Revise vcpkg_from_sourceforge

[dg0yt]

This PR was triggered by seeing three (!) slashes in

Downloading soxr-0.1.3-Source.tar.xz, trying https://sourceforge.net/projects/soxr/files///soxr-0.1.3-Source.tar.xz/download

This is because of https://cmake.org/cmake/help/v4.1/policy/CMP0174.html#policy:CMP0174

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I hoped this PR would fix SF download issues #48210, #48203, #48201 on Github Actions. But actually I still do not achieve success in a GHA test project.

Installing 2/2 soxr:x64-linux@0.1.3#8...
Building soxr:x64-linux@0.1.3#8...
Downloading soxr-0.1.3-Source.tar.xz, trying https://sourceforge.net/projects/soxr/files/soxr-0.1.3-Source.tar.xz/download
Trying https://sourceforge.net/projects/soxr/files/soxr-0.1.3-Source.tar.xz/download?use_mirror=cfhcable
...
error: curl: (22) The requested URL returned error: 403
...

A manual local wget or curl -L succeeds, as do local vcpkg builds.
Edit: The SF issue seems to be resolved now.