Vendir allows to use unsafe paths in configuration

This issue was initially [discussed in Slack](https://kubernetes.slack.com/archives/CH8KCCKA5/p1703443931837529).

**What steps did you take:**

Here is a shell script to reproduce the issue. It runs commands during `docker build`, so you can safely run it, as it doesn't change any files on the host system.

```shell
#!/usr/bin/env bash

cat <<'EOF' | docker build --progress=plain --file=- .
FROM alpine:20231219
ARG TARGETOS=linux
ARG TARGETARCH=amd64
ARG VENDIR_VERSION=v0.38.0
RUN wget -q \
https://github.com/carvel-dev/vendir/releases/download/${VENDIR_VERSION}/vendir-${TARGETOS}-${TARGETARCH} \
-O /usr/bin/vendir
RUN chmod +x /usr/bin/vendir

WORKDIR /some/safe/place
RUN pwd
RUN ls -l /usr
RUN cat <