Precommand assignments after `env` should be allowed to be quoted

[danielshahaf]

See #627 (comment) and the following comments.

[danielshahaf]

For greppability purposes, quoting @phy1729 analysis from there:

sudo env "PATH=$PATH"
First word is sudo which is a known precommand, so next word will be parsed as a command.
Next word is env which is a known precommand, so next word will be parsed as a command.
Next word is "PATH=$PATH". There's a bug in z-sy-h that when parsing a command ($this_word == *:start:*) an assignment is recognized even if it's no long the true start of a command (i.e. after a precommand), so command foo=bar echo baz parses foo=bar as an assignment and echo as the command even though command will error as foo=bar isn't a command. However in the case of env this mostly is desired. env being a command allows for quoted arguments just like any other, but assignments at the beginning of a command must not be quoted.