fix: BUN_OPTIONS bare flags getting trailing whitespace (#26464)

## Summary

Fix a bug in `appendOptionsEnv` where bare flags (no `=`) that aren't
the last option get a trailing space appended, causing the argument
parser to not recognize them.

For example, `BUN_OPTIONS="--cpu-prof --cpu-prof-dir=profiles"` would
parse `--cpu-prof` as `"--cpu-prof "` (trailing space), so CPU profiling
was never enabled.

## Root Cause

When `appendOptionsEnv` encounters a `--flag` followed by whitespace, it
advances past the whitespace looking for a possible quoted value (e.g.
`--flag "quoted"`). If no quote is found and there's no `=`, it falls
through without resetting `j`, so the emitted argument includes the
trailing whitespace.

## Fix

Save `end_of_flag = j` after scanning the flag name. Add an `else`
branch that resets `j = end_of_flag` when no value (quote or `=`) is
found after the whitespace. This is a 3-line change.

Also fixes a separate bug in `BunCPUProfiler.zig` where `--cpu-prof-dir`
with an absolute path would hit a debug assertion (`path.append` on an
already-rooted path with an absolute input). Changed to `path.join`
which handles both relative and absolute paths correctly.

## Tests

- `test/cli/env/bun-options.test.ts`: Two new tests verifying
`--cpu-prof --cpu-prof-dir=<abs-path>` produces a `.cpuprofile` file,
for both normal and standalone compiled executables.

test/cli/env/bun-options.test.ts

@@ -1,6 +1,7 @@
 import { spawnSync } from "bun";
 import { describe, expect, test } from "bun:test";
-import { bunEnv, bunExe } from "../../harness";
+import { readdirSync } from "fs";
+import { bunEnv, bunExe, tempDir } from "harness";
 
 describe("BUN_OPTIONS environment variable", () => {
   test("basic usage - passes options to bun command", () => {
@@ -56,6 +57,61 @@ describe("BUN_OPTIONS environment variable", () => {
     expect(result.stdout.toString()).toContain("COMMAND LINE");
   });
 
+  test("bare flag before flag with value is recognized", () => {
+    // Bare flags (no =) that aren't the last option must not get a
+    // trailing space appended. --cpu-prof is a bare flag; --cpu-prof-dir
+    // uses = syntax. If --cpu-prof isn't recognized, no profile is written.
+    using dir = tempDir("bun-options-cpu-prof", {});
+
+    const result = spawnSync({
+      cmd: [bunExe(), "-e", "1"],
+      env: {
+        ...bunEnv,
+        BUN_OPTIONS: `--cpu-prof --cpu-prof-dir=${dir}`,
+      },
+    });
+
+    expect(result.exitCode).toBe(0);
+
+    // --cpu-prof should have produced a .cpuprofile file in the dir
+    const files = readdirSync(String(dir));
+    const cpuProfiles = files.filter((f: string) => f.endsWith(".cpuprofile"));
+    expect(cpuProfiles.length).toBeGreaterThanOrEqual(1);
+  });
+
+  test("bare flag before flag with value is recognized (standalone executable)", () => {
+    // Same test as above but with a compiled standalone executable.
+    using dir = tempDir("bun-options-cpu-prof-compile", {
+      "entry.ts": "console.log('ok');",
+    });
+
+    const exePath = String(dir) + "/app";
+    const profDir = String(dir) + "/profiles";
+
+    // Compile
+    const build = spawnSync({
+      cmd: [bunExe(), "build", "--compile", String(dir) + "/entry.ts", "--outfile", exePath],
+      env: bunEnv,
+    });
+    expect(build.exitCode).toBe(0);
+
+    // Run with BUN_OPTIONS
+    const result = spawnSync({
+      cmd: [exePath],
+      env: {
+        ...bunEnv,
+        BUN_OPTIONS: `--cpu-prof --cpu-prof-dir=${profDir}`,
+      },
+    });
+
+    expect(result.stdout.toString()).toContain("ok");
+    expect(result.exitCode).toBe(0);
+
+    const files = readdirSync(profDir);
+    const cpuProfiles = files.filter((f: string) => f.endsWith(".cpuprofile"));
+    expect(cpuProfiles.length).toBeGreaterThanOrEqual(1);
+  });
+
   test("empty BUN_OPTIONS - should work normally", () => {
     const result = spawnSync({
       cmd: [bunExe(), "--print='NORMAL'"],