Fix bug which prevents Lakeshore blocking_t from exiting

docs/changes/newsfragments/7796.improved_driver

@@ -0,0 +1,4 @@
+Fixed bug in Lakeshore driver that was causing blocking_t
+to periodically hang indefinitely: a 0.5 seconds post delay was added to
+setpoint parameter to ensure that the setpoint value propagated to the
+instrument when the set of the setpoint parameter returns.

src/qcodes/instrument_drivers/Lakeshore/lakeshore_base.py

@@ -236,6 +236,7 @@ def __init__(
             get_parser=float,
             set_cmd=f"SETP {output_index}, {{}}",
             get_cmd=f"SETP? {output_index}",
+            post_delay=0.5,  # getting setpoint too soon after setting may fetch previous setpoint
         )
         """
         The value of the setpoint in the preferred units of the control loop sensor