more reliable target directory estimation

[peterwilli]

Currently, llama-cpp-rs fails with "not found" when compiling a project using a different profile.

For example, having this in our cargo toml:

[profile.dist]
lto = true
inherits = "release"
codegen-units = 1
panic = "abort"
strip = "debuginfo"

[profile.release]
opt-level = 3

Then compile like this: cargo build --profile=dist will fail because PROFILE is always "release"

The new way just goes back 3 steps which will always be the target (source: https://stackoverflow.com/a/73603419).