Fix pip bootstrap URL

[thewilsonator]

This error was hit on #3683

[thewilsonator]

Azure linux/ android gives

g++-multilib : Depends: gcc-multilib (>= 4:5.3.1-1ubuntu1) but it is not going to be installed
                Depends: g++-5-multilib (>= 5.3.1-3~) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

noticed on dlang/dmd too, unrelated to this.

[Geod24]

#3680

[kinke]

Thx; I wanted to keep on experimenting with #3680 for Travis AArch64 but lost all interest in the meantime. Merged now, so at least this annoyance is resolved; for Azure Linux, where MS manages to f*ck this up regularly, we'll probably have to wait for a fix on their end. Edit: actions/runner-images#2917

[thewilsonator]

Thanks!