misc: use .venv by default instead of venv (#3629)

I didn't want to have this discussion before we merged uv, but it
definitely feels like it really prefers `.venv`, and makes lots of
things easier if this name is used. How do we feel about using this name
by default? It can still easily be overridden locally, but will change
to .venv by default for everyone.

.github/workflows/ci-core.yml

@@ -15,8 +15,6 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
-    env:
-      VENV_DIR: .venv
     strategy:
       matrix:
         python-version: ['3.10', '3.11', '3.12', '3.13']

.github/workflows/ci-mlir.yml

@@ -19,7 +19,6 @@ jobs:
         python-version: ['3.10', '3.11', '3.12']
 
     env:
-      VENV_DIR: .venv
       LLVM_SYMBOLIZER_PATH: /usr/lib/llvm-11/bin/llvm-symbolizer
       MLIR-Version: d401987fe349a87c53fe25829215b080b70c0c1a
     steps:

.github/workflows/ci-notebooks.yml

@@ -15,8 +15,6 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
-    env:
-      VENV_DIR: .venv
     strategy:
       matrix:
         python-version: ['3.10', '3.11', '3.12']

.github/workflows/ci-pyright-fails.yml

@@ -19,7 +19,6 @@ jobs:
         python-version: ["3.10", "3.11", "3.12"]
 
     env:
-      VENV_DIR: .venv
       PYRIGHT_VERSION: 1.0
 
     steps:

Makefile

@@ -5,7 +5,7 @@ MAKEFLAGS += --no-builtin-variables
 COVERAGE_FILE ?= .coverage
 
 # allow overriding the name of the venv directory
-VENV_DIR ?= venv
+VENV_DIR ?= .venv
 UV_PROJECT_ENVIRONMENT=${VENV_DIR}
 
 # allow overriding which extras are installed