fix: close leaked file handles in config and token key loading

[fereidani]

Hi,

I've built a static analyzer for Go called Ghoul, and I ran it against the cloudflared codebase. It flagged a couple of file handles that are opened but never closed, which can leak file descriptors over the lifetime of the process (and across config reloads). This PR closes both.

Ghoul related reports:

config/configuration.go:422:2: [resource-leak] resource from os.Open allocated at config/configuration.go:413 never closed (confidence: 6.4/10, CWE-404)
  --> config/configuration.go:413:25: source:      resource allocated by os.Open
token/encrypt.go:159:3: [resource-leak] resource from os.Open allocated at token/encrypt.go:150 never closed (confidence: 6.4/10, CWE-404)
  --> token/encrypt.go:150:19: source:      resource allocated by os.Open
token/encrypt.go:164:2: [resource-leak] resource from os.Open allocated at token/encrypt.go:150 never closed (confidence: 6.4/10, CWE-404)
  --> token/encrypt.go:150:19: source:      resource allocated by os.Open

Changes:

• config/configuration.go: the strict-mode re-parse in ReadConfigFile opens the config file a second time without closing it. Added a deferred close so the handle is released on every path.
• token/encrypt.go: Encrypter.fetchKey opens the key file but never closes it on any return path. Added a deferred close.

Both use the project's defer func() { _ = file.Close() }() convention.