Enable adding Ubuntu toolchain PPA by default

[Flamefire]

The default in Boost.CI will be false but some jobs still rely on this, so keep it for now the default.

[mborland]

We have a number of failures that look like:

...updated 283 targets...
+ uname
+ [[ Linux == Linux ]]
.drone/drone.sh: 43: [[: not found
+ echo ==================================> BEFORE_SCRIPT
==================================> BEFORE_SCRIPT
==================================> SCRIPT
+ . /drone/src/.drone/before-script.sh
+ echo ==================================> SCRIPT
+ echo using gcc : : g++-11 : <cxxflags>-std=gnu++17  ;
+ cd libs/config/test
+ ../../../b2 print_config_info print_math_info toolset=gcc
/drone/boost-root/tools/build/src/tools/common.jam:1101: in common.find-compiler from module common
error: toolset gcc initialization:
error: provided command '"g++-11"' not found
error: initialized from /root/user-config.jam:1
/drone/boost-root/tools/build/src/tools/gcc.jam:163: in gcc.init from module gcc
/drone/boost-root/tools/build/src/build/toolset.jam:44: in toolset.using from module toolset
/drone/boost-root/tools/build/src/build/project.jam:1323: in using from module project-rules
/root/user-config.jam:1: in modules.load from module user-config
/drone/boost-root/tools/build/src/build-system.jam:258: in load-config from module build-system
/drone/boost-root/tools/build/src/build-system.jam:468: in load-configuration-files from module build-system
/drone/boost-root/tools/build/src/build-system.jam:628: in module scope from module build-system

Same thing shows on your Multiprecision PR

[Flamefire]

2 issues:

.drone/drone.sh: 25: [: develop: unexpected operator
+ [[ develop == */master ]]
.drone/drone.sh: 25: [[: not found

It looks like for some reason bash isn't used? No idea here

error: provided command '"g++-11"' not found

The launchpad PPA is not reachable, which is why it won't be added by default anymore: Too much flakiness.
I'd suggest to use the updated .drone.star from Boost.CI, then this PR can be closed too.