fix(inputs.opcua): Assign node id correctly

[sspaink]

resolve: #11719

The ID was being set to the local copy of "node" in the loop, in order for the to be saved so it can be used later in the code: https://github.com/influxdata/telegraf/blob/master/plugins/inputs/opcua/opcua.go#L516 it needs to be assigned using the index so that the original value gets updated. More context of this behavior: https://stackoverflow.com/questions/61605727/why-does-normal-for-loop-allows-assigning-value-to-struct-fields-while-for-range

[telegraf-tiger]

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DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_i386.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz
		static_linux_amd64.tar.gz