fix: include the gRPC gem in the installation tests

[michal-kazmierczak]

This PR includes the minimum necessary change to ensure the installation test passes with the unfinished gRPC instrumentation.

Further discussion is needed on how the installation script could be included earlier in the build process to prevent unexpected failures.

Before:

❯ ./run.sh
Installing latest version of published gems and running tests
Fetching gem metadata from https://rubygems.org/........
Resolving dependencies...
Bundle complete! 38 Gemfile dependencies, 144 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.

(...)

        class Client < ::GRPC::ClientInterceptor
                       ^^^^^^
(...)
rake aborted!
(...)
Tasks: TOP => default => test
(See full trace by running task with --trace)

After:

❯ ./run.sh
Installing latest version of published gems and running tests
Fetching gem metadata from https://rubygems.org/........
Resolving dependencies...
Bundle complete! 39 Gemfile dependencies, 147 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
(...)
Run options: --seed 43820

# Running:

.

Finished in 0.000559s, 1788.9088 runs/s, 1788.9088 assertions/s.

1 runs, 1 assertions, 0 failures, 0 errors, 0 skips