build: remove quotes from rust-version — heartwood

The script would include the quotes and so running build/build would result in:

$ build/build
Running build..
✓ Radicle key already in ssh-agent
Building Radicle 1.2.0..
Creating archive of repository at e9cf91… in build/heartwood-e9cf91….tar.gz..
Building image (radicle-build-1.2.0)..
[1/2] STEP 1/17: FROM rust:"1.85"-alpine3.20 AS builder
Error: creating build container: parsing reference "rust:\"1.85\"-alpine3.20": invalid reference format

 #!/bin/sh
 if ! version=$(grep -m 1 -oP "channel\s*=\s*\K(.*)" rust-toolchain.toml); then
 if ! version=$(grep -m 1 -oP "channel\s*=\s*\K(.*)" rust-toolchain.toml | tr -d '"'); then
   echo "fatal: no rust version found via rust-toolchain.toml" >&2 ; exit 1
 fi