binman: bintool: run 'apt-get update -y' on first invocation of apt_install()

binman: bintool: run 'apt-get update -y' on first invocation of apt_install()

'apt-get update -y' may be required to make sure that the list of
packages is up-to-date and that the subsequent 'apt-get install'
operations can find the requested packages.

Fixes the following CI error:

 Fetch: zstd
 - trying method: binary download
 - sudo apt-get install -y zstd
 Exception: Error 100 running 'sudo apt-get install -y zstd': E: Unable to locate package zstd

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Bryan Brattlof <bb@ti.com>