builder/services.go: Update proxyd flag
builder/services.go: Update proxyd flag

Required change to match
https://vanadium-review.googlesource.com/#/c/5330/

MultiPart: 2/2
Change-Id: Ifd108947764c20a9a2c43596cb8461637efb5eb1

vanadium/playground,vanadium/playground,vanadium/playground,vanadium/playground