maturin breaks with version 0.12.7 and 0.12.8 due to inexistent pyproject.toml

[tropxy]

Bug Description

maturin breaks, because expects a pyproject.toml

	  maturin build \
	  -i /tmp/python3/bin/python3 \
	  --target=armv7-unknown-linux-gnueabihf"
💥 maturin failed
  Caused by: pyproject.toml is invalid
  Caused by: Couldn't find pyproject.toml at /project/pyproject.toml
  Caused by: No such file or directory (os error 2)
make: *** [build] Error 1

If maturin v0.12.6 is used, the problem does not occur

Your Python version (python -V)

Tested with Python 3.8.2 and Python 3.10

Your pip version (pip -V)

pip 21.1.3

What bindings you're using

pyo3

Does cargo build work?

• Yes, it works

If on windows, have you checked that you aren't accidentally using unix path (those with the forward slash /)?

• Yes

Steps to Reproduce

1. install maturin 0.12.8
2. run maturin build

[messense]

Do you have a reproducible project?

[tropxy]

Do you have a reproducible project?

Sorry, just saw this now. And I also see you have already merged a solution, thanks ;)