Add offline build support with vendored dependencies

[swaroop-osec]

This PR adds support for offline builds by implementing a vendored dependencies workflow. When the --offline flag is passed to the build command, the tool will:

1. Run cargo vendor to create a vendor directory with all dependencies
2. Create a .cargo/config.toml file with vendored sources configuration
3. Configure the build to use local dependencies instead of fetching from the internet

Usage:

solana-verify build -- --offline

The implementation:

• Adds a new setup_offline_build function to handle the offline build setup
• Modifies the existing build function to check for the --offline flag and trigger the offline build flow.
• Maintains compatibility with existing build workflows
• Preserves all existing functionality when offline mode is not used

Testing:

• Verified that the offline build works correctly with vendored dependencies
• Tested with Drift Vaults program.

solana-verify verify-from-repo https://github.com/drift-labs/drift-vaults/ --library-name drift_vaults --commit-hash 969abfe150873b315a160fa42b001ec9fcda9152 -b ellipsislabs/solana:1.16.6 --program-id vAuLTsyrvSfZRuRB3XgvkPwNGgYSs9YRYymVebLKoxR -- --offline

[ngundotra]

LGTM. Only consideration - is there ever a scenario where users might need to delete the vendored config?

[swaroop-osec]

LGTM. Only consideration - is there ever a scenario where users might need to delete the vendored config?

I don't think so. If they don't want to have vendored dependencies they can omit --offline flag.