Change pipx binary directory (#17)

Defining the `PIPX_BIN_DIR` environment variable allows us to change where pipx stores binaries at. Previously this was `~/.local/bin` but with this PR the directory has been changed to `/usr/local/bin`

linux/Dockerfile

@@ -41,6 +41,7 @@ RUN apt-get install -y zip clang lldb lld clangd \
 	llvm-$LLVM_VER-dev libpolly-$LLVM_VER-dev
 
 # Install llvm-lit, which we use for correctness tests
+ENV PIPX_BIN_DIR=/usr/local/bin
 RUN python3 -m venv /opt/venv \
 	&& /opt/venv/bin/python -m pip install --upgrade pip \
 	&& /opt/venv/bin/python -m pip install pipx \