[vcpkg-bootstrap.sh] Enable finding gcc 9

[ras0219-msft]

Should fix #6411 @acgetchell

[MVoz]

why produce versions? in * unix it is easier to configure

example

sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-6 30
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-7 50
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-8 60
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-9 80

sudo update-alternatives --install /usr/bin/cc cc /usr/bin/gcc 30

sudo update-alternatives --install /usr/bin/c++ c++ /usr/bin/g++ 30

sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-9 60

did not watch, there MacOS, is not familiar with this OS

[acgetchell]

In an environment like Travis-CI with sudo: false then sudo update-alternatives doesn't work.

[MVoz]

in * UNIX, the sudo command is administrative privileges

update-alternatives
https://wiki.debian.org/DebianAlternatives

[ras0219-msft]

I'm not able to think of a good way to use "*" in this case -- if you have a more concrete idea I'd be happy to see a PR!

[MVoz]

It seems to me that the user should do this on his own machine, just like installing MSVC
once in settings

and here to register in the DOC how to configure

But that's my personal opinion ..