Machine-readable `--format yaml` produces text output while `--format json` doesn't

[cmaglie]

Describe the problem

Machine-readable formatting produces text output.

To reproduce

$ arduino-cli core update-index --format yaml
Downloading index: package_index.tar.bz2 downloaded                                                                                 
Downloading index: package_esp8266com_index.json downloaded       
$

Expected behavior

$ arduino-cli core update-index --format yaml
$

Arduino CLI version

nightly

Operating system

N/A

Operating system version

all

Additional context

No response

Issue checklist

• I searched for previous reports in the issue tracker
• I verified the problem still occurs when using the nightly build
• My report contains all necessary details

[cmaglie]

Related:

$ arduino-cli compile -b alice:avr:alice --show-properties ~/testdata/bare_minimum
Warning: Board alice:avr:alice doesn't define a 'build.board' preference. Auto-set to: AVR_ALICE
_id=alice
build.arch=AVR
build.board=AVR_ALICE
build.core=arduino
build.core.path=/tmp/go-testsuite1676247329/Arduino15/packages/alice/hardware/avr/1.0.0/cores/arduino
build.fqbn=alice:avr:alice
[...cut...]

The output of compile --show-properties has the spurious warning: Warning: Board alice:avr:alice doesn't define a 'build.board' preference. Auto-set to: xxxxx
Since this message is not strictly related to the output it should be removed or sent to stderr.