proxy: Fix build with golang 1.13 to 1.15

MR !102 (merged) broke proxy build with golang 1.13 to 1.15, so this MR fixes that

PS: Maybe bump the minimal required Golang version to the one used in Debian stable, aka 1.15?
PPS: Also, it seems building the Snowflake client with Go 1.13 fails with some go mod error line: https://gitlab.torproject.org/KokaKiwi/snowflake/-/jobs/184448#L218