According to this summary document on house.gov, they're not. I thought this was very interesting considering all the hubbub in the House of Representatives about the FBI agents being or not being whistleblowers.

Who Is(n’t) Covered?

Most executive branch employees, former employees, and applicants fall within the WPA’s protections because they are part of the
merit system that governs the federal civil service under Title 5 of the U.S. Code. Employees of the Government Publishing Office, a
Legislative Branch agency, are also covered. (5 U.S.C. § 2302(a)(2)(C)).
However, some executive branch employees are excluded from the WPA’s protections, including (but not limited to):
❖ Political appointees (e.g. federal inspectors general)
❖ Uniformed military service members
❖ Noncareer Senior Executive Service employees
❖ Employees of the 17 different intelligence community “elements” and the FBI
❖ Members of the U.S. Public Health Service Commissioned Corps
❖ Officers of the National Oceanic and Atmospheric Association (NOAA) Commissioned Corps
❖ Employees of the U.S. Postal Service