ci(.radicle/ambient.yaml): add Radicle bin directory to path

I previously installed Radicle from a `deb`, which meant `rad` is in
`/usr/bin`, and thus in the default `$PATH`. But now I install it with
the Radicle installer, and need to add `/root/.radicle/bin` to `$PATH`.

Signed-off-by: Lars Wirzenius <liw@liw.fi>

.radicle/ambient.yaml

@@ -30,7 +30,10 @@ plan:
       export CARGO_TARGET_DIR=/workspace/cache
       export CARGO_HOME=/workspace/deps
       export HOME=/root
-      export PATH="/root/.cargo/bin:$PATH"
+      export PATH="/root/.cargo/bin:/root/.radicle/bin:$PATH"
+
+      # Check that `rad` is there.
+      rad --help >/dev/null || (echo rad is not there; exit 1)
 
       cargo doc --workspace --no-deps