refactor(services/azblob): use OsEnv instead of StaticEnv hack

[YuangGao]

Which issue does this PR close?

• Related to (azure-storage): wire builder methods through credential providers opendal-reqsign#756 and bug: azdls backend doesn't load Azure credentials from environment variables #7224.

Rationale for this change

azblob's builder was injecting user-supplied account_name / account_key / sas_token into reqsign by translating them into fake env vars stuffed into a StaticEnv. The same values were also pushed into the credential chain via StaticCredentialProvider::new_shared_key / new_sas_token (see lines 393-406 in the old code), so the fake-env path was redundant — only push_front actually fired, because it has higher priority in the chain.

The StaticEnv hack also previously masked OS env vars (see #7224 root cause for azdls); the same anti-pattern in azblob has no functional benefit.

What changes are included in this PR?

Switch to the s3 pattern:

• .with_env(StaticEnv { ... }) -> .with_env(OsEnv).
• Drop the envs.insert("AZBLOB_ACCOUNT_NAME", ...) / AZBLOB_ACCOUNT_KEY / AZURE_STORAGE_SAS_TOKEN translation block.
• Keep the existing StaticCredentialProvider::new_shared_key / new_sas_token push_front chain (already the correct API).
• Keep the base64 validation for account_key.
• Drop now-unused reqsign_core::Env / StaticEnv imports.

Are there any user-facing changes?

No behavior change for users who set credentials via opendal config

[Xuanwo]

Thank you!