util: fix parseEnv incorrectly splitting multiple ‘=‘ in value

Previously, parseEnv would create multiple environment
variables if a single line contained multiple ‘=‘ characters
(e.g. A=B=C would become { A: ‘B=C’, B: ‘C’ }).
This commit ensures that only the first ‘=‘ is used as
the key-value delimiter, and the rest of the line is treated
as the value.

Fixes: https://github.com/nodejs/node/issues/57411
PR-URL: https://github.com/nodejs/node/pull/57421
Reviewed-By: Daniel Lemire <daniel@lemire.me>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

benchmark/fixtures/valid.env

@@ -6,6 +6,8 @@ BASIC=basic
 
 # previous line intentionally left blank
 AFTER_LINE=after_line
+A="B=C"
+B=C=D
 EMPTY=
 EMPTY_SINGLE_QUOTES=''
 EMPTY_DOUBLE_QUOTES=""