Use static AWS credentials for elasticsearch indexer if set

What does this MR do and why?

A customer using OpenSearch for Advanced Search is getting this error

"exception.class":"Aws::STS::Errors::AccessDenied",
"exception.message":"Not authorized to perform sts:AssumeRoleWithWebIdentity",
"exception.backtrace":[
    "ee/lib/gitlab/elastic/client.rb:78:in `block in aws_credential_provider'",
    "ee/lib/gitlab/elastic/client.rb:77:in `aws_credential_provider'",
    "ee/lib/gitlab/elastic/indexer.rb:214:in `build_aws_credentials_env'",
    "ee/lib/gitlab/elastic/indexer.rb:150:in `build_envvars'",
    "ee/lib/gitlab/elastic/indexer.rb:102:in `run_indexer!'",
    "ee/lib/gitlab/elastic/indexer.rb:76:in `block in run'",
    "ee/lib/gitlab/elastic/indexer.rb:57:in `each'",

Even when AWS credentials are set.

To fix, we should first check if static credentials are set and use that.

Fixes https://gitlab.com/gitlab-com/enablement-sub-department/section-enable-request-for-help/-/issues/80

MR acceptance checklist

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How to set up and validate locally

1. On the Search admin page, set credentials in the OpenSearch (doesn't have to be valid credentials).

2. Add a puts in ee/lib/gitlab/elastic/indexer.rb line 153: puts vars

3. Run Gitlab::Elastic::Indexer.new(Project.first).run in a console

4. See that the credentials you set are passed as ENV vars to the indexer