fix: use placeholder for artifact/cache server address in man page

[xingzihai]

Problem

The default values for --artifact-server-addr and --cache-server-addr were calling GetOutboundIP() during flag definition, which embedded the actual outbound IP address in the generated man page.

This causes:

• Reproducibility issue: Building on different machines produces different man pages (different IP addresses recorded)
• Privacy concern: Users IP address is recorded in the man page

From the issue:

│ ├── usr/share/man/man1/act.1.gz
│ │ │ ├── act.1
│ │ │ │ @@ -26,15 +26,15 @@
│ │ │ │ -B--artifact-server-addrP="162.55.28.166"
│ │ │ │ +B--artifact-server-addrP="147.75.84.133"

Solution

Changed the flag defaults to empty string and added runtime auto-detection logic in newRunCommand to compute the outbound IP when the values are empty.

This ensures:

• Man pages are reproducible (show empty string as default)
• Privacy is preserved (no IP in documentation)
• Runtime behavior is preserved (IP is auto-detected when needed)

Changes

1. Changed --artifact-server-addr flag default from common.GetOutboundIP().String() to empty string
2. Changed --cache-server-addr flag default from common.GetOutboundIP().String() to empty string
3. Added auto-detection logic in newRunCommand to compute the IP if either address is empty
4. Updated flag descriptions to explain auto-detection

Fixes: #5894