fix: pass through unexpected format strings

Uses a regex-based solution instead of str.format

unit_test/utils_test.py

@@ -0,0 +1,36 @@
+from cibuildwheel.util import format_safe, prepare_command
+
+
+def test_format_safe():
+    assert format_safe("{wheel}", wheel="filename.whl") == "filename.whl"
+    assert format_safe("command \\{wheel}", wheel="filename.whl") == "command {wheel}"
+    assert format_safe("{command \\{wheel}}", wheel="filename.whl") == "{command {wheel}}"
+
+    # check unmatched brackets
+    assert format_safe("{command {wheel}", wheel="filename.whl") == "{command filename.whl"
+
+    # check positional-style arguments i.e. {}
+    assert (
+        format_safe("find . -name  * -exec ls -a {} \\;", project="/project")
+        == "find . -name  * -exec ls -a {} \\;"
+    )
+
+
+def test_prepare_command():
+    assert prepare_command("python -m {project}", project="project") == "python -m project"
+    assert prepare_command("python -m {something}", project="project") == "python -m {something}"
+    assert (
+        prepare_command("python -m {something.abc}", project="project")
+        == "python -m {something.abc}"
+    )
+
+    assert (
+        prepare_command("python -m {something.abc[4]:3f}", project="project")
+        == "python -m {something.abc[4]:3f}"
+    )
+
+    # test some unusual syntax that used to trip up the str.format approach
+    assert (
+        prepare_command("{a}{a,b}{b:.2e}{c}{d%s}{e:3}{f[0]}", a="42", b="3.14159")
+        == "42{a,b}{b:.2e}{c}{d%s}{e:3}{f[0]}"
+    )