[Web, Windows] Fix build failure on Windows

[AThousandShips]

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Follow up to:

• https://github.com/godotengine/godot/pull/104458

In PowerShell (at least, haven't tried with other shells) the pipe characters are not interpreted as part of the expression, leading to:

[ 67%] em++: error: error parsing "-s" setting "PTHREAD_POOL_SIZE='Module[emscriptenPoolSize]": unclosed quoted string. expected final character to be "'" and length to be greater than 1 in "'"
[ 68%] '8'' is not recognized as an internal or external command, operable program or batch file.

Swapping the quote characters solves this issue (though I don't know how to confirm that the result is parsed correctly in either case)