examples: improve robustness of deploy scripts

- The scripts now work within arbitrary nix-shells.
  Previously, they failed when run from nix shells other than
  `examples/shell.nix`.

- The scripts now work from arbitrary working dirs.

examples/deploy-qemu-vm.sh

@@ -11,10 +11,12 @@ set -euo pipefail
 # MAKE SURE TO REPLACE the SSH identity file if you use this script for
 # anything serious.
 
-if [[ ! -v IN_NIX_SHELL ]]; then
+if [[ ! -v NIX_BITCOIN_EXAMPLES_DIR ]]; then
     echo "Running script in nix shell env..."
     cd "${BASH_SOURCE[0]%/*}"
     exec nix-shell --run "./${BASH_SOURCE[0]##*/} $*"
+else
+    cd "$NIX_BITCOIN_EXAMPLES_DIR"
 fi
 
 source qemu-vm/run-vm.sh