tor-snowflake: read the Go version from the module root

Snowflake is a single Go module with go.mod at the repository root, not
under proxy/. The install read /opt/tor-snowflake/proxy/go.mod, which
has never existed, so grep failed and the version came out empty. Go
was then fetched from

  https://go.dev/dl/go.linux-amd64.tar.gz

with no version in the name, and the install aborted at that download.

Read the directive with a single awk instead of grep piped into awk.
The pipeline tripped catch_errors, which sets -Ee with pipefail and an
ERR trap, the moment the file was missing; that is the "exit code 2
while executing command awk" line that preceded the download failure.

Fall back to latest when the file cannot be read, so a future upstream
move of go.mod costs a Go version that is newer than the one pinned
rather than a failed build. setup_go already resolves latest, and also
resolves a bare major.minor to its newest patch, so a go directive
without a patch level stays fine.

Verified against v2.14.1: go.mod sits at the root and declares go
1.24.0, and the awk yields 1.24.0 from it and latest from a missing
file without tripping the ERR trap.

install/tor-snowflake-install.sh

@@ -16,7 +16,8 @@ update_os
 
 msg_info "Building Snowflake"
 GITLAB_URL="https://gitlab.torproject.org" fetch_and_deploy_gl_release "tor-snowflake" "tpo/anti-censorship/pluggable-transports/snowflake" "tarball"
-GO_VERSION="$(grep -m1 '^go ' /opt/tor-snowflake/proxy/go.mod | awk '{print $2}')" setup_go
+GO_VERSION="$(awk '/^go /{print $2; exit}' /opt/tor-snowflake/go.mod 2>/dev/null || true)"
+GO_VERSION="${GO_VERSION:-latest}" setup_go
 cd /opt/tor-snowflake/proxy
 $STD go build -o snowflake-proxy .
 msg_ok "Built Snowflake Proxy"