Installer will override clang on ubuntu even when clang is already installed

[Caviarion]

Describe the bug
Installer will override clang on ubuntu even when clang-8 is already installed.

To Reproduce
Env:

• Ubuntu 16.04
• Run apt install clang-8 lld-8

Steps to reproduce the behavior:

1. Install rust and cargo normally
2. cargo build works fine
3. Install fleet with curl -L get.fleet.rs | sh
4. Both cargo build and fleet build failed with clang: error: invalid linker name in argument '-fuse-ld=lld'
5. clang --version suggests that it has been degraded to 3.8

Additional context

• It seems that the installer only checks whether clang package is installed, leaving the fact that user may have already installed another version of clang or even build manually.

[GrinderDeveloper]

Should be handled, mate!

[VarunPotti]

This has been fixed in the development versions of the installer. We now require you to follow a more manual way of installing clang/lld.