Remove baked bpy-default.sh/bpy-dev.sh; prefer system pixi

bpy-default.sh and bpy-dev.sh were saved 'pixi shell-hook' output from
another host, with PIXI_EXE and PATH pointing into a personal home that
doesn't exist on the consoles. bpy-env.sh now runs the hook at runtime
(--as-is, ~0.2 s) for its shared-env fallback instead of sourcing
bpy-default.sh; nothing else used either file.

Scripts that call pixi now prefer the system /usr/bin/pixi and fall back
to /sf/bernina/bin/pixi (e.g. on ra, which has no system pixi).

create-python-install

@@ -43,7 +43,8 @@
 set -euo pipefail
 
 SOURCE_DIR="/sf/bernina/applications/python"
-PIXI_BIN="/sf/bernina/bin/pixi"
+PIXI_BIN="/usr/bin/pixi"                                # console (system) pixi first
+[ -x "$PIXI_BIN" ] || PIXI_BIN="/sf/bernina/bin/pixi"   # fallback, e.g. on ra (RHEL8)
 EXP_DIR="/sf/bernina/exp"
 DATA_DIR="/sf/bernina/data"
 LINK_DIR_NAME="conda_envs"   # must not be "envs", see above