.tool-env generation

[hagl]

The code that generates .tool-env fails to insert line separators between the individual export statements.

On my machine the generated file looks as follows and the OPENOCD_SCRIPTS value gets the export of the next var appended.

export OPENOCD_SCRIPTS=/nix/store/...-idf-v5.5.2/openocd-esp32/share/openocd/scriptsexport ESP_ROM_ELF_DIR=/nix/store/...-esp-rom-elfs-esp-idf-v5.5.2/

Fixes #127

[mornepousse]

Confirmed this fixes #127, tested on x86_64-linux with esp-idf-full.

Root cause is visible in the generated file: wc -l on .tool-env returns 0,
i.e. every export ends up concatenated on a single unterminated line, which
is why the last path picks up the next statement's export.

Before:

$ echo $OPENOCD_SCRIPTS
/nix/store/...-openocd-esp32-esp-idf-v5.5.2/openocd-esp32/share/openocd/scriptsexport
$ [ -d "$OPENOCD_SCRIPTS" ] && echo OUI || echo NON
NON

With this PR applied:

$ echo $OPENOCD_SCRIPTS
/nix/store/...-openocd-esp32-esp-idf-v5.5.2/openocd-esp32/share/openocd/scripts
$ ls "$OPENOCD_SCRIPTS/board" | grep -c '\.cfg$'
442

So openocd board configs such as esp32s3-builtin.cfg are reachable again.