Is it okay to store enviornment variables as global variables by reading from file once?

I have a config.env file which has a SecretKey, I need the value of this key everytime I want to parse/read from JWT (token).
So, I have thought of 2 possible ways:

1. Store the SecretKey as a global variable when the code(.env file reading) executes so it can be used later
   I don't know if that will beat the entire purpose of having an .env file

2. Read from file everytime I want to parse the token. (which will be almost every API call)

So, 1 or 2?
Or something else entirely?

P.S. Just to be clear I am not asking if I can replace env with global, but whether I can store the SecretKey as a global variable once the code is deployed/executed.

Yes, you can store it, why couldn't you?

And yes, it's a (very) good idea not to read / parse the file every time you need it, that would (could) perceptibly make your API calls slower (depending on what they actually do).

Whether you need or should use a global variable is another question. Generally, global variables are discouraged. This may be a good exception, although you may choose to store it in another ways, e.g. in some kind of application context, or in fields of the handler that implement your endpoints.

Regarding item 1

In general, global variables should be avoided because makes the code less reusable and more difficult to test. Let's say it is coupled to that global variable. It also limits the code to be used with one only value at a time: What if there is a need to use two different SecretKey values?
To decouple the code, the configuration value can be injected (for example using a struct or a closure).

For example, the following functionality is decoupled from global variables and it could be a package/module ready to reuse:

type MyApiCaller struct {
    SecretKey string
}

func (m *MyApiCaller) DoSomething() {
    fmt.Println("Do something with", m.SecretKey)
}

Now it can be used anywhere, even as multiple global variables:

var Caller1 = &MyApiCaller{"secret1"}
var Caller2 = &MyApiCaller{"secret2"}

It is also easy to test because the state is not shared through a global variable.

Regarding item 2

Reading the configuration file each time the value is needed will probably limit the program performance as disk access is very expensive. The lateral advantage is that the program would get the latest config without restart.