[ruby/rubygems] Replace instance method look up in plugin installer

`Gem::Installer.instance_methods(false).include?(:generate_plugins)` is
63x slower than `Gem::Installer.method_defined?(:generate_plugins)` in a
microbenchmark. The latter is a direct lookup, whereas the former will
create an array, which will be slower.

```ruby
require "benchmark/ips"

Benchmark.ips do |x|
  x.report "instance_methods" do
    Gem::Installer.instance_methods(false).include?(:generate_plugins)
  end

  x.report "method_defined" do
    Gem::Installer.method_defined?(:generate_plugins)
  end

  x.compare!
end
```

```
$ ruby -I lib/ benchmark_methods.rb
ruby 3.4.4 (2025-05-14 revision https://github.com/ruby/rubygems/commit/a38531fd3f) +PRISM [arm64-darwin23]
Warming up --------------------------------------
    instance_methods    58.449k i/100ms
      method_defined     3.375M i/100ms
Calculating -------------------------------------
    instance_methods    541.874k (± 5.7%) i/s    (1.85 μs/i) -      2.747M in   5.087825s
      method_defined     34.263M (± 1.1%) i/s   (29.19 ns/i) -    172.135M in   5.024524s

Comparison:
      method_defined: 34263189.1 i/s
    instance_methods:   541874.3 i/s - 63.23x  slower
```

This does not make much difference in an overall benchmark or profile,
but this is more idiomatic Ruby than the prior code.

https://github.com/ruby/rubygems/commit/49dec52cb2

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