Replace subshell with exec

[webflo]

Related to #6137

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This replaces the sub-process with exec, so the signals are treated correctly. But it won't fix the issue if drush is invoked from ./vendor/bin/drush (the composer wrapper script, creates a sub-process too).

[weitzman]

LGTM. Lets see if anyone else has thoughts ... Please remove from draft when you are satisfied.

[Chi-teck]

./vendor/bin/drush is a primary endpoint for Drush. Given it does not handle signals you will have to update your Drush launcher, CI scripts, crontabs, etc. Everything that calls Drush...

[Chi-teck]

Also these shell wrappers increase the number of processes. Drush 13.3 for each command starts 3 processes. I have a project that runs ~30 Drush workers in background, which produces 90 processes. With this PR it'll be reduced to 60 processes I guess, but it is still not good.

[Chi-teck]

Filed a ticket for Composer
composer/composer#12164

[webflo]

@weitzman Done, Composer adopted the same solution.

[webflo]

@Chi-teck Could you do a test with composer self-update --snapshot? This has the new exec code

[Chi-teck]

Confirm. With this PR and updated composer no additional shell processes are created. Also signals are handled correctly.