build(python/adbc_driver_manager): enable conda support properly

[lidavidm]

Closes #3483.

[amoeba]

Tested locally and this works fine. +1

I tested by building the python package with _ADBC_IS_CONDA=1 and having CONDA_PREFIX=foo,

-- >8 -- 
	not set: ADBC_DRIVER_PATH
	additional search path: /Users/bryce/src/apache/arrow-adbc/.venv/etc/adbc
	Conda prefix: foo/etc/adbc/drivers        <--- this is good
	user config dir: /Users/bryce/Library/Application Support/ADBC/Drivers
	does not exist: /Library/Application Support/ADBC/Drivers