Add bring-your-own-buffer versions of os.lookup_env and os.get_env

And make `core:terminal` use it so that `core:log` can be imported with `-default-to-nil-allocator`,
in which the actual allocator is set up in `main()`.

Windows was tricky because of the utf-8 <> utf-16 conversion, so we use some temporary stack buffers for that purpose,
limiting the non-allocating version there to 512 utf-16 characters each for the key and environment value.

In general the value is (obviously) limited to the size of the supplied buffer, and a `.Buffer_Full` error is returned
if that buffer is insufficient. If the key is not found, the procedure returns `.Env_Var_Not_Found`.

TODO:
- Factor out buffer-backed utf8 + utf16 conversion to `core:sys/util` to more easily apply this pattern.
- Add similar `lookup_env` and `get_env` procedures to `core:os/os2`.

Fixes #5336

core/os/os_linux.odin

@@ -946,7 +946,7 @@ access :: proc(path: string, mask: int) -> (bool, Error) {
 }
 
 @(require_results)
-lookup_env :: proc(key: string, allocator := context.allocator) -> (value: string, found: bool) {
+lookup_env_alloc :: proc(key: string, allocator := context.allocator) -> (value: string, found: bool) {
 	runtime.DEFAULT_TEMP_ALLOCATOR_TEMP_GUARD(ignore = context.temp_allocator == allocator)
 	path_str := strings.clone_to_cstring(key, context.temp_allocator)
 	// NOTE(tetra): Lifetime of 'cstr' is unclear, but _unix_free(cstr) segfaults.
@@ -958,11 +958,39 @@ lookup_env :: proc(key: string, allocator := context.allocator) -> (value: strin
 }
 
 @(require_results)
-get_env :: proc(key: string, allocator := context.allocator) -> (value: string) {
+lookup_env_buffer :: proc(buf: []u8, key: string) -> (value: string, err: Error) {
+	if len(key) + 1 > len(buf) {
+		return "", .Buffer_Full
+	} else {
+		copy(buf, key)
+	}
+
+	if value = string(_unix_getenv(cstring(raw_data(buf)))); value == "" {
+		return "", .Env_Var_Not_Found
+	} else {
+		if len(value) > len(buf) {
+			return "", .Buffer_Full
+		} else {
+			copy(buf, value)
+			return string(buf[:len(value)]), nil
+		}
+	}
+}
+lookup_env :: proc{lookup_env_alloc, lookup_env_buffer}
+
+@(require_results)
+get_env_alloc :: proc(key: string, allocator := context.allocator) -> (value: string) {
 	value, _ = lookup_env(key, allocator)
 	return
 }
 
+@(require_results)
+get_env_buf :: proc(buf: []u8, key: string) -> (value: string) {
+	value, _ = lookup_env(buf, key)
+	return
+}
+get_env :: proc{get_env_alloc, get_env_buf}
+
 set_env :: proc(key, value: string) -> Error {
 	runtime.DEFAULT_TEMP_ALLOCATOR_TEMP_GUARD()
 	key_cstring := strings.clone_to_cstring(key, context.temp_allocator)