Fix two errors reported by golangci-lint

[yuanchen8911]

This PR fixes the following two errors reported by golang-lint.

$ golangci-lint run ./...
WARN [lintersdb] The linter named "megacheck" is deprecated. It has been split into: gosimple, staticcheck, unused.
pkg/utils/k8s_config_test.go:113:15: Error return value of `os.Unsetenv` is not checked (errcheck)
			os.Unsetenv("KUBECONFIG")
			           ^
pkg/utils/k8s_config_test.go:117:20: Error return value of `os.Remove` is not checked (errcheck)
				defer os.Remove(f.Name())
				               ^
pkg/utils/k8s_config_test.go:130:20: Error return value of `os.Remove` is not checked (errcheck)
				defer os.Remove(f.Name())
				               ^
pkg/config/config.go:58:15: G304: Potential file inclusion via variable (gosec)
	data, err := os.ReadFile(path)
	             ^

[shinae-woo]

Thanks!