So, a few things...
_ready function is fired immediately after the node is added to the scene tree. So, it'll run before the
_input even is ever processed. So, even if your code was right (which it isn't), the variable wouldn't be set at the time the
_ready function fires.
Regarding, how to access the same variable in multiple functions... The easiest way is to make the variable "global" to the script. That way, you can access it from anywhere in the local script.
To do that, just define the variable outside of any function. Then just reference it as needed from anywhere else in the script. So, for example:
var my_global # define the variable outside of any function
my_global = 10 # assign a value to the global variable
print(my_global) # prints "10"
my_global += 5 # change the value
print(my_global) # prints 15