fix: mustBeMetaMask should not detect brave as MetaMask

[gomesalexandre]

Brave Wallet's window.ethereum sets isMetaMask to true, hence using the mustBeMetaMask option here will falsely detect Brave as MM, see:

https://github.com/brave/brave-core/blob/ae4c9dab752c588fc7b5921863f862777a19589d/components/brave_wallet/renderer/js_ethereum_provider.cc#L254-L256

This is a non-standard behavior of Brave as opposed to most MetaMask impersonators setting isMetaMask to false, hence requires explicit handling. Thankfully, a isBraveWallet property is also exposed (https://github.com/brave/brave-core/pull/12794), which we can check for to guard against wrongly detecting Brave as MM.