feat: add environment variable support for in-flight checksumming

[leodido]

Description

Adds support for the LEEWAY_ENABLE_IN_FLIGHT_CHECKSUMS environment variable to enable in-flight checksumming without requiring CLI flags. This makes it easier to configure in CI environments while maintaining full backward compatibility.

Merge after #243

Key Changes:

• Add LEEWAY_ENABLE_IN_FLIGHT_CHECKSUMS environment variable constant
• Implement environment variable as default with CLI flag override in getBuildOpts()
• Follow the same pattern as existing SLSA environment variables (EnvvarSLSACacheVerification)
• Use cmd.Flags().Changed() to distinguish explicit flag setting from default values
• Add comprehensive test coverage for all environment variable and flag combinations
• Update help documentation to include the new environment variable

Behavior:

• Environment variable serves as the default value when CLI flag is not explicitly set
• CLI flag always takes precedence when explicitly provided (maintains backward compatibility)
• No behavior change for existing users unless they set the environment variable

Related Issue(s)

Addresses the need for easier CI configuration of the in-flight checksumming feature.

Fixes https://linear.app/ona-team/issue/CLC-1960/implement-in-flight-checksumming-leeway

How to test

Manual Testing

# Test 1: Environment variable enables feature
export LEEWAY_ENABLE_IN_FLIGHT_CHECKSUMS=true
leeway build --dry-run <target>
# Should show in-flight checksumming enabled

# Test 2: CLI flag overrides environment variable
export LEEWAY_ENABLE_IN_FLIGHT_CHECKSUMS=false
leeway build --in-flight-checksums --dry-run <target>
# Should show in-flight checksumming enabled (flag wins)

# Test 3: Default behavior unchanged
unset LEEWAY_ENABLE_IN_FLIGHT_CHECKSUMS
leeway build --dry-run <target>
# Should show in-flight checksumming disabled

Automated Testing

# Run the new environment variable tests
go test -run TestInFlightChecksumsEnvironmentVariable ./cmd/ -v

# Run all build-related tests
go test ./cmd/ -v