Minor nit to not re-enable sql_log_bin + free disk space on gh runner before running tests

[shayancanonical]

Issue

1. We are re-enabling sql_log_bin in flush_mysql_logs() when there is no need - the variable is only valid for the session
2. We are running out of disk space when running CI (see the following run)

Solution

1. Prevent re-enabling the variable
2. Free disk space on GH runner before running the tests

[codecov]

Codecov Report

Merging #339 (89583f3) into main (1f72f1b) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #339   +/-   ##
=======================================
  Coverage   64.50%   64.50%           
=======================================
  Files          17       17           
  Lines        3113     3113           
  Branches      413      413           
=======================================
  Hits         2008     2008           
  Misses        979      979           
  Partials      126      126           

Files	Coverage Δ
lib/charms/mysql/v0/mysql.py	73.22% <100.00%> (ø)

[carlcsaposs-canonical]

Suggested change

	- name: Free disk space
	run: |
	echo "Free disk space before cleanup"
	df -T
	# free space in the runner
	- name: Free up disk space
	run: |
	echo "Free disk space before cleanup"
	df -T
	# From https://github.com/actions/runner-images/issues/2840#issuecomment-790492173

nit

[shayancanonical]

will commit nit in the next PR to avoid rerunning tests