msi: refactor exectuion bat files

[daipom]

• Add entire local scope to prevent changing global env vars
• Remove unnecessary enabledelayedexpansion
• Remove unnecessary /

fluentd.bat

Before: FLUENT_PACKAGE_VERSION was passed to Fluentd process.
After: FLUENT_PACKAGE_VERSION is not passed to Fluentd process.

fluent-gem.bat

Previously, it changed the environment variables unintentionally.

Before

> ruby -e "ENV['PATH'].split(';').select{|v| v.include?('fluent')}.map{|v| p v}"
"C:\\opt\\fluent\\"
"C:\\opt\\fluent\\bin"

> fluent-gem --version
3.4.19

> ruby -e "ENV['PATH'].split(';').select{|v| v.include?('fluent')}.map{|v| p v}"
"C:\\opt\\fluent\\bin"
"C:\\opt\\fluent\\"
"C:\\opt\\fluent\\bin"

After

> ruby -e "ENV['PATH'].split(';').select{|v| v.include?('fluent')}.map{|v| p v}"
"C:\\opt\\fluent\\"
"C:\\opt\\fluent\\bin"

> fluent-gem --version
3.4.19

> ruby -e "ENV['PATH'].split(';').select{|v| v.include?('fluent')}.map{|v| p v}"
"C:\\opt\\fluent\\"
"C:\\opt\\fluent\\bin"

[kenhys]

LGTM.

I've overlooked these variable scopes.