--build_manual_tests applies to non-tests

### Description of the problem:

The `--build_manual_tests` flag builds targets tagged "manual" but which are not test targets. This behavior seems inconsistent with the [documentation](https://docs.bazel.build/versions/3.0.0/command-line-reference.html#flag--build_manual_tests), which seems to imply that the flag applies specifically to test targets:

>Forces test targets tagged 'manual' to be built. 'manual' tests are excluded from processing. This option forces them to be built (but not executed).

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Consider a workspace consisting of three files:

BUILD:
```
java_binary(
name = "bin",
srcs = glob(["Manual.java"]),
main_class = "Manual",
tags = ["manual"],
)
```

Manual.java:
```
public class Manual {
public static void main(String[] args) {
System.out.println("Hello");
}
}
```

WORKSPACE:
(empty)

Steps:
1. Edit Manual.java
2. Run `bazel build --build_manual_tests ...`

Expected outcome: no targets are built.

Actual outcome: The `bin` target is built:
```
$ bazel build --build_manual_tests --subcommands=pretty_print ...

INFO: Analyzed target //:bin (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
SUBCOMMAND: # //:bin [action 'Building bin.jar (1 source file)', configuration: dbadffe9d0533bf51e2ba2aac08b5435397cdb67cd0b74b86e24900343122bb0]
(cd /home/sebelino/.cache/bazel/_bazel_sebelino/c3c7a33107b6cbc76f7cc84608df699f/execroot/__main__ && \
exec env - \
LC_CTYPE=en_US.UTF-8 \
PATH=/usr/local/bin:/usr/local/sbin:/opt/google-cloud-sdk/bin:/home/sebelino/scripts:/home/sebelino/bin:/usr/bin:/home/sebelino/code/go/bin:/opt/ida/bin:/opt/android-sdk/tools:/opt/android-sdk/tools/bin:/usr/lib/jvm/default/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:/opt/sonar-scanner/bin \
external/remotejdk11_linux/bin/java \
-XX:+UseParallelOldGC \
-XX:-CompactStrings \
'--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED' \
'--add-exports=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED' \
'--add-exports=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED' \
'--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED' \
'--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED' \
'--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED' \
'--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED' \
'--add-opens=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED' \
'--patch-module=java.compiler=external/remote_java_tools_linux/java_tools/java_compiler.jar' \
'--patch-module=jdk.compiler=external/remote_java_tools_linux/java_tools/jdk_compiler.jar' \
'--add-opens=java.base/java.nio=ALL-UNNAMED' \
'--add-opens=java.base/java.lang=ALL-UNNAMED' \
-jar \
external/remote_java_tools_linux/java_tools/JavaBuilder_deploy.jar \
@bazel-out/k8-fastbuild/bin/bin.jar-0.params \
@bazel-out/k8-fastbuild/bin/bin.jar-1.params)
Target //:bin up-to-date:
bazel-bin/bin.jar
bazel-bin/bin
INFO: Elapsed time: 0.130s, Critical Path: 0.05s
INFO: 1 process: 1 worker.
INFO: Build completed successfully, 2 total actions
```

### What operating system are you running Bazel on?

Linux 5.5.13-arch2-1

### What's the output of `bazel info release`?

release 3.0.0

Start by reproducing the reported workspace with a manual-tagged java_binary and `bazel build --build_manual_tests ...`, then trace the `--build_manual_tests` target-filtering entry point. Done means manual non-test targets are excluded while manual test targets retain the documented build behavior, with coverage for both cases.

Written by the indexing model from the issue text.