test: use t.Setenv to set env vars

[Juneezee]

Overview

Starting from Go 1.17, we can use t.Setenv to set environment variable in test. The environment variable is automatically restored to its original value when the test and all its subtests complete. This ensures that each test does not start with leftover environment variables from previous completed tests.

Reference: https://pkg.go.dev/testing#T.Setenv

How to Test

$ go test ./...
?   	sniffer	[no test files]
?   	sniffer/internal/version	[no test files]
?   	sniffer/pkg/context	[no test files]
?   	sniffer/pkg/ebpfev	[no test files]
?   	sniffer/pkg/ebpfev/v1	[no test files]
?   	sniffer/pkg/storageclient	[no test files]
ok  	sniffer/internal/validator	0.014s
ok  	sniffer/pkg/config	0.010s
ok  	sniffer/pkg/config/v1	0.011s
ok  	sniffer/pkg/conthandler	25.045s
ok  	sniffer/pkg/conthandler/v1	0.027s
ok  	sniffer/pkg/ebpfeng	0.015s
ok  	sniffer/pkg/event_data_storage	5.020s
ok  	sniffer/pkg/sbom	0.037s
ok  	sniffer/pkg/sbom/v1	0.043s
ok  	sniffer/pkg/utils	0.013s