make: let .coverage be the standard coverage file

Python coverage tooling such as pytest-cov and coverage.py expects that
.coverage is a sqlite file with the coverage results.

TICS by default expects .coverage to be a directory containing
cobertura.xml.  TICS is configurable, however, so let's move to let the
python coverage tooling have the defaults expected here, instead of
expecting coverage tools to treat Subiquity special.

Author: dbungert

.coveragerc

@@ -11,7 +11,7 @@ DRYRUN?=--dry-run --bootloader uefi --machine-config examples/machines/simple.js
 	--postinst-hooks-dir examples/postinst.d/
 UNITTESTARGS?=
 COVERAGEARGS:=--cov=subiquity --cov=subiquitycore --cov=console_conf
-COVERAGEARGS+=--cov-report xml:.coverage/cobertura.xml
+COVERAGEARGS+=--cov-report xml:.coverage.xml
 export PYTHONPATH
 CWD := $(shell pwd)