Tweak GC settings in pipeline config

What does this MR do?

This MR sets a few Ruby GC env variables in order to speed up the CI suite time a bit.

Settings:

ENV_VAR	description	note
RUBY_GC_HEAP_INIT_SLOTS	Initial allocation slots. Introduced in Ruby 2.1, default: 10000.	Less allocations on boot if increased
RUBY_GC_MALLOC_LIMIT	The initial limit of young generation allocation from the malloc-family. GC will start when this limit is reached. Default: 16MB	Increasing this value reduces the number of GC runs
RUBY_GC_MALLOC_LIMIT_MAX	The maximum limit of young generation allocation from malloc before GC starts. Prevents excessive malloc growth due to RUBY_GC_MALLOC_LIMIT_GROWTH_FACTOR. Introduced in Ruby 2.1, default: 32MB.	2 x RUBY_GC_MALLOC_LIMIT
RUBY_GC_MALLOC_LIMIT_GROWTH_FACTOR	Increases the limit of young generation malloc calls, reducing GC frequency but increasing malloc growth until RUBY_GC_MALLOC_LIMIT_MAX is reached. Introduced in Ruby 2.1, default: 1.4, minimum: 1.0 (no growth)
RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR	Perform a full GC when the number of old objects is more than R * N, where R is this factor and N is the number of old objects after the 3 last full GC. Introduced in Ruby 2.1.1, default: 2.0	Defer full GC if increased
RUBY_GC_OLDMALLOC_LIMIT	The initial limit of old generation allocation from malloc, a full GC will start when this limit is reached. Introduced in Ruby 2.1, default: 16MB
RUBY_GC_OLDMALLOC_LIMIT_MAX	The maximum limit of old generation allocation from malloc before a full GC starts. Prevents excessive malloc growth due to RUBY_GC_OLDMALLOC_LIMIT_GROWTH_FACTOR. Introduced in Ruby 2.1, default: 128MB

Source: https://github.com/ruby/ruby/blob/master/man/ruby.1

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team