build-tf-a.sh: disable keyring while building

TF-A build added poetry as mandatory dependency, required to run tools
like `cot-dt2c`. TF-A's poetry environment installing is initiated
during build. However, poetry tries to use the keyring which triggers a
GUI prompt asking for user password which is undesirable. To avoid this,
set `PYTHON_KEYRING_BACKEND` to null, following the recommended method:
https://github.com/jaraco/keyring#disabling-keyring



Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>