Validate the uniqueness of pipeline variables

What does this MR do?

Prevents Projects::PipelinesController#create from returning 500 errors when there are duplicate pipeline variables: https://log.gprd.gitlab.net/goto/2998692616fc89c188b46ff726630523

We have a uniqueness validation on the pipeline variables model, but that doesn't work because the validations are executed on all the records before we start inserting them:

[8] pry(main)> pipeline = Ci::Pipeline.last.dup; nil
  Ci::Pipeline Load (0.5ms)  SELECT "ci_pipelines".* FROM "ci_pipelines" ORDER BY "ci_pipelines"."id" DESC LIMIT 1 
=> nil
[9] pry(main)> pipeline.iid = nil
=> nil
[10] pry(main)> pipeline.variables_attributes = [{ key: 'a', value: 'a' }, { key: 'a', value: 'b' }]
=> [{:key=>"a", :value=>"a"}, {:key=>"a", :value=>"b"}]
[11] pry(main)> pipeline.save
  TRANSACTION (0.2ms)  BEGIN
  Ci::PipelineVariable Exists? (0.3ms)  SELECT 1 AS one FROM "ci_pipeline_variables" WHERE "ci_pipeline_variables"."key" = 'a' AND "ci_pipeline_variables"."pipeline_id" IS NULL LIMIT 1
  Ci::PipelineVariable Exists? (0.3ms)  SELECT 1 AS one FROM "ci_pipeline_variables" WHERE "ci_pipeline_variables"."key" = 'a' AND "ci_pipeline_variables"."pipeline_id" IS NULL LIMIT 1
  Project Load (0.8ms)  SELECT "projects".* FROM "projects" WHERE "projects"."id" = 13 LIMIT 1
  Feature::FlipperGate Load (2.8ms)  SELECT "feature_gates".* FROM "feature_gates" WHERE "feature_gates"."feature_key" = 'generate_iids_without_explicit_locking'
  InternalId Load (6.7ms)  SELECT "internal_ids".* FROM "internal_ids" WHERE "internal_ids"."project_id" = 13 AND "internal_ids"."usage" = 5 LIMIT 1 
  InternalId Load (0.3ms)  SELECT "internal_ids".* FROM "internal_ids" WHERE "internal_ids"."id" = 23 LIMIT 1 FOR UPDATE 
  InternalId Update (0.3ms)  UPDATE "internal_ids" SET "last_value" = 9 WHERE "internal_ids"."id" = 23 
  Ci::Pipeline Create (4.9ms)  INSERT INTO "ci_pipelines" ("ref", "sha", "before_sha", "created_at", "updated_at", "project_id", "status", "started_at", "finished_at", "duration", "user_id", "lock_version", "source", "config_source", "protected", "iid", "ci_ref_id") VALUES ('main', 'f72367b97387a8f1b7b381a3521f699d83708f56', '0000000000000000000000000000000000000000', '2021-08-09 13:21:41.844206', '2021-08-09 13:21:41.844206', 13, 'failed', '2021-07-22 12:47:43.206252', '2021-07-22 12:49:11.598515', 40, 1, 5, 2, 1, TRUE, 9, 4) RETURNING "id" 
  Ci::PipelineVariable Create (1.5ms)  INSERT INTO "ci_pipeline_variables" ("key", "value", "encrypted_value", "encrypted_value_salt", "encrypted_value_iv", "pipeline_id") VALUES ('a', NULL, 'l/M+tii2dBlbyKHqO9Ug9g==
', '_CCbDl3MfR5AvEyv6/ByjTg==
', 'kZsukkHfZNHe/T9N8AvuIg==
', 36) RETURNING "id"
  Ci::PipelineVariable Create (0.5ms)  INSERT INTO "ci_pipeline_variables" ("key", "value", "encrypted_value", "encrypted_value_salt", "encrypted_value_iv", "pipeline_id") VALUES ('a', NULL, 'HL0/vjvMoDj11wY+UGJLVw==
', '_qH/yOP0PY8Dx+7hK4a3zrg==
', 'o6WYLvZxHoQ4P1HdnA6+5w==
', 36) RETURNING "id" 
  TRANSACTION (0.2ms)  ROLLBACK
ActiveRecord::RecordNotUnique: PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_ci_pipeline_variables_on_pipeline_id_and_key"
DETAIL:  Key (pipeline_id, key)=(36, a) already exists.

from /Users/marius/.gem/ruby/2.7.2/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/postgresql_adapter.rb:672:in `exec_params'
Caused by PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_ci_pipeline_variables_on_pipeline_id_and_key"
DETAIL:  Key (pipeline_id, key)=(36, a) already exists.

from /Users/marius/.gem/ruby/2.7.2/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/postgresql_adapter.rb:672:in `exec_params'

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