docker_run_tests.sh: stop using cmake

Use test_driver directly instead of CMake in the docker tests.

Deal with the read-only "/fish-source" by exporting
"CARGO_TARGET_DIR=$HOME/fish-build".  It seems correct to also inject this
environment variable into the interactive debugging shells.  Add some logging
to make this override more obvious to the user.

Adopt "build_tools/check.sh", because that defines the full set of checks
that we (eventually) want to run in CI.

In particular, this will also run "tests/checks/po-files-up-to-date.fish"
which "cargo b && cargo t && tests/test_driver.py" does not, due to the
REQUIRES clause.

Since most docker images have some lints/warnings today, disable those for
now. Use "docker_run_tests.sh --lint" to override. The default may be changed
in future.

docker/focal.Dockerfile

@@ -9,11 +9,9 @@ RUN apt-get update \
   && apt-get -y install \
     build-essential \
     cargo \
-    cmake \
     gettext \
     git \
     locales \
-    ninja-build \
     pkg-config \
     python3 \
     python3-pexpect \
@@ -35,4 +33,6 @@ WORKDIR /home/fishuser
 
 COPY fish_run_tests.sh /
 
+ENV FISH_CHECK_LINT=false
+
 CMD /fish_run_tests.sh