Tighten the version grep in vendor script

We just added `rust-version = ` to the `secp256k1-sys` manifest, doing
so causes a grep statement from the vendor script to match this line -
we don't want that.

Tighten up the grep statement by only matching on `version` at the start
of the line.

Author: tcharding

secp256k1-sys/vendor-libsecp.sh

@@ -8,7 +8,7 @@ else
     SECP_VENDOR_GIT_ROOT="$(realpath "$SECP_VENDOR_GIT_ROOT")"
 fi
 SECP_SYS="$SECP_VENDOR_GIT_ROOT"/secp256k1-sys
-DEFAULT_VERSION_CODE=$(grep version "$SECP_SYS/Cargo.toml" | sed 's/\./_/g' | sed 's/.*"\(.*\)".*/\1/')
+DEFAULT_VERSION_CODE=$(grep "^version" "$SECP_SYS/Cargo.toml" | sed 's/\./_/g' | sed 's/.*"\(.*\)".*/\1/')
 DEFAULT_DEPEND_DIR="$SECP_SYS/depend"
 DEFAULT_SECP_REPO=https://github.com/bitcoin-core/secp256k1.git