Extend detection of needed values in build-from-source instructions for Linux

Hello. In the build-from-source instructions for Linux at https://osquery.readthedocs.io/en/latest/development/building/, 'export ARCH=$(uname -m)' illustrates an attempt to detect a value in order to avoid manual specification in command-line. That's an efficient practice. The same practice could be applied to the OSQUERY TOOLCHAIN program release version as well.

So doing,

• the URL in wget https://github.com/osquery/osquery-toolchain/releases/download/1.3.0/osquery-toolchain-1.3.0-${ARCH}.tar.xz would be replaced by wget https://github.com/osquery/osquery-toolchain/releases/latest/osquery-toolchain-*-${ARCH}.tar.xz.
• 'sudo tar -xvf osquery-toolchain-1.3.0-${ARCH}.tar.xz -C /usr/local' would be replaced by 'sudo tar -xvf $(ls osquery-toolchain-*-*.tar.xz) -C /usr/local'.